Editor's Note: (Peter Bergen is CNN's national security analyst, a vice president at New America and a professor of practice at Arizona State University. He is the author of "Manhunt: The Ten-Year Search for bin Laden -- From 9/11 to Abbottabad." He is the lead author of the just-released New America policy paper "ISIS in the West: The New Faces of Extremism", co-authored by Courtney Schuster, a program associate at New America, and David Sterman a senior program associate there.)
(CNN) On Friday, France had its 9/11.
At least 129 people were killed at multiple locations in Paris, including a concert hall, a soccer stadium and a popular restaurant, the kinds of venues that ordinary Parisians flock to on a Friday night.
At, or near, these venues the attackers deployed a mix of terrorist tactics, including suicide attackers, an assault using more than one gunman willing to fight to the death, hostage-taking and bombings.
In the years after 9/11, we have seen various forms of this terrible news story play out: the multiple bombs on trains in Madrid that killed 191 in 2004, the four suicide bombings in London that killed 52 commuters in 2005, and the 2008 attacks in Mumbai, India, by 10 gunmen who killed 166.
The attackers in Paris seemed to have learned lessons from all these attacks. (By the way, this is also the case of U.S. school shootings in which the perpetrators study the tactics of those who have gone before them.)
World reacts to Paris attacks
President Barack Obama, French President Francois Hollande, second from right, and Paris Mayor Anne Hidalgo arrive at the Bataclan, site of one of the Paris terrorists attacks, to pay their respects to the victims after Obama arrived in town for the COP21 climate change conference early on Monday, November 30, in Paris.
The Eiffel Tower in Paris is illuminated in the French national colors on Monday, November 16. Displays of support for the French people were evident at landmarks around the globe after the deadly terrorist attacks in Paris on Friday, November 13.
People hold hands as they observe a minute of silence in Lyon, France, on November 16, three days after the Paris attacks. A minute of silence was observed throughout the country in memory of the victims of the country's deadliest violence since World War II.
French President Francois Hollande, center, flanked by French Prime Minister Manuel Valls, right, and French Education Minister Najat Vallaud-Belkacem, center left, stands among students during a minute of silence in the courtyard of the Sorbonne University in Paris on November 16.
A large crowd gathers to lay flowers and candles in front of the Carillon restaurant in Paris on Sunday, November 15.
A man sits next to candles lit as homage to the victims of the deadly attacks in Paris at a square in Rio de Janeiro on November 15.
People light candles in tribute to the Paris victims on November 15 in Budapest, Hungary.
People gather outside Notre Dame Cathedral in Paris on November 15 for a national service for the victims of the city's terror attacks.
People write messages on the ground at Place de la Republique in Paris on November 15.
People pray during a candlelight vigil for victims of the Paris attacks at a church in Islamabad, Pakistan, on November 15.
French golfer Gregory Bourdy passes a peace symbol for the Paris victims during the BMW Shanghai Masters tournament November 15 in Shanghai, China.
A man offers a prayer in memory of victims of the Paris attacks at the French Embassy in Tokyo on November 15.
A woman holds a candle atop a miniature replica of the Eiffel Tower during a candlelight vigil Saturday, November 14, in Vancouver, British Columbia.
Front pages of Japanese newspapers in Tokyo show coverage and photos of the Paris attacks on November 14.
An electronic billboard on a canal in Milan, Italy reads, in French, "I'm Paris," on November 14.
The Eiffel Tower stands dark as a mourning gesture on November 14, in Paris. More than 125 people were killed in a series of coordinated attacks in Paris on Friday. People around the world reacted in horror to the deadly terrorist assaults.
Lithuanians hold a candlelight vigil in front of the French Embassy in Vilnius, Lithuania, on November 14.
Thousands gather in London's Trafalgar Square for a candlelit vigil on November 14 to honor the victims of the Paris attacks.
A woman lights candles at a memorial near the Bataclan theater in Paris on November 14.
A man places a candle in front of Le Carillon cafe in Paris on November 14.
A woman holds a French flag during a gathering in Stockholm, Sweden, on November 14.
Nancy Acevedo prays for France during the opening prayer for the Sunshine Summit being held at Rosen Shingle Creek in Orlando, Florida on November 14.
French soldiers of the United Nations' interim forces in Lebanon observe the national flag at half-staff at the contingent headquarters in the village of Deir Kifa on November 14.
A couple surveys the signature sails of the Sydney Opera House lit in the colors of the French flag in Sydney on November 14.
A woman places flowers in front of the French Consulate in St. Petersburg, Russia, on November 14.
Candles are lit in Hong Kong on November 14 to remember the scores who died in France.
A woman lights a candle outside the French Consulate in Barcelona, Spain, on November 14.
Britain's Prince Charles expresses solidarity with France at a birthday barbecue in his honor near Perth, Australia, on November 14.
The French national flag flutters at half-staff on November 14 at its embassy in Beijing.
Dutch Prime Minister Mark Rutte after a speech on November 14 in The Hague following the attacks.
Japanese Prime Minister Shinzo Abe becomes emotional after his speech on the French attacks during the opening ceremony of a Japanese garden in Istanbul, Turkey, on November 14.
A woman mourns outside Le Carillon bar in the 10th district of Paris on November 14. The attackers ruthlessly sought out soft targets where people were getting their weekends underway.
People lay flowers outside the French Embassy in Moscow on November 14.
Mourners gather outside Le Carillon bar in the 10th district of Paris on November 14. "We were listening to music when we heard what we thought were the sounds of firecrackers," a doctor from a nearby hospital who was drinking in the bar with colleagues told Le Monde. "A few moments later, it was a scene straight out of a war. Blood everywhere."
People attend a vigil outside the French Consulate in Montreal. Prime Minister Justin Trudeau offered "all of Canada's support" to France on Friday, November 13, in the wake of the attacks.
Police show a heightened presence in Times Square in New York on November 13, following the terrorist attacks in Paris.
People light candles at a vigil outside the French Consulate in Montreal on November 13.
University of Nevada, Las Vegas, fans observe a moment of silence for the victims of the terrorist attacks in Paris before a basketball game November 13.
The house lights are shut off and scoreboard dark as Boston Celtics players pause for a moment of silence for the Paris victims before an NBA basketball game against the Atlanta Hawks in Boston on November 13.
People light candles at a vigil outside the French Consulate in Montreal on November 13.
French President Francois Hollande blames ISIS for the attack, and the terror group has claimed responsibility. According to French prosecutors, one of the attackers who has been identified is a French national known to police, and a Syrian passport was found on the body of one of the other attackers. CNN reports that this militant was posing as a Syrian refugee. It is still early in the investigation, but according to CNN French affiliate BFM, six of the Paris attackers spent time in Syria.
Until now, French citizen Mehdi Nemmouche is the only case of a Western fighter in Syria accused of returning to conduct a deadly terror attack -- the May 24, 2014, shooting at the Jewish Museum in Brussels, Belgium, that left four people dead. Nemmouche had served time in a French prison, and he had an assault rifle when he was arrested in France. A French journalist held by ISIS reportedly has identified Nemmouche as one of the group's alleged torturers. Nemmouche has been extradited to Belgium, where he awaits trial.
Returning militants from Syria are a worrying potential source of terror attacks. And two major factors place Europe at far greater risk of "returnee" violence from veterans of the Syrian conflict than is the case in the United States: the much larger number of European militants who have gone to fight in Syria and the existence of more developed jihadist networks in Europe.
Before 'Jihadi John,' Mohammed Emwazi in London terror network
France has supplied more fighters to the Syrian conflict than any other Western country. In September, Prime Minister Manuel Valls told Parliament that 1,800 French citizens have been involved in jihadist networks worldwide -- almost all of whom were drawn to the Syrian war. Nine months earlier, Interior Minister Bernard Cazeneuve estimated that 185 militants had returned to France from Syria. Of those who had returned, he said 82 were in jail and 36 were under other forms of judicial control.
German security services report that 720 Germans have left for Syria, and they estimate that 100 have been killed there, while another 180 have returned to Germany.
Last year, the Belgian Foreign Ministry released figures that up to 350 Belgians had left to fight in Syria.
Upward of 700 British citizens have left for Syria, with about half estimated to have returned to the United Kingdom, according to British officials.
In January, Australian Foreign Minister Julie Bishop placed the number of Australians fighting abroad at 180, with 20 having died in Syria.
So who exactly are the estimated 4,500 Westerners who have been drawn to join ISIS and other militant groups in Syria?
To provide some answers to that question, New America collected information about 466 individuals from 25 Western countries who have been reported by credible news sources as having left their home countries to join ISIS or other Sunni jihadist groups in Syria or Iraq.
The Western fighters drawn to Syria and Iraq represent a new demographic profile, quite different than that of other Western militants who fought in Afghanistan in the 1980s or Bosnia in the 1990s.
First, women are represented in unprecedented numbers. One in seven of the militants in New America's data set are women. Women were rarely, if at all, represented in previous jihadist conflicts.
While Western women are not going to fight in the war in Syria, they are playing supporting roles, often marrying front-line fighters and sometimes working as police officers.
They are women like Sally Jones, 44, from the United Kingdom, who took her 10-year-old son to Syria in 2013, and Emilie Konig, 31, one of the first women to leave for Syria, who left France and her two children behind in 2012 to join her husband there. The U.S. State Department says both women have encouraged terrorist attacks in their native countries, and it officially designated both of them as terrorists in September.
Second, the recruits are young. The average age of Western volunteers drawn to the Syrian jihad is 24. For female recruits, the average age is 21. Almost a fifth are teenagers, more than a third of whom are female.
New America has documented an astonishing 80 cases of Western teenagers who have traveled to the war in Syria. More than a third of these teenagers are girls. Hans-Georg Maassen, the head of Germany's domestic security agency, said, for instance, in March that nine female German teens had left for Syria.
That same month, ISIS released a video of a French boy shooting a Palestinian hostage in the forehead.
Increased security following Paris attacks
People go through a security checkpoint to attend a Mass in homage to victims of the Paris terror attacks at Notre Dame cathedral in Paris on Sunday, November 15. French President Francois Hollande declared a state of emergency after the attacks in Paris on Friday, November 13, and said border security has been ramped up. The terrorist group ISIS claimed responsibility for the attacks.
French Prime Minister Manuel Valls, center, speaks with police forces with French Interior minister Bernard Cazeneuve, left, and Defence Minister Jean-Yves Le Drian at the Gare du Nord railway station in Paris on November 15 about security measures after the attacks.
French police secure the perimeter after panic broke out among mourners who paid their respects at the attack sites at restaurant Le Petit Cambodge and the Carillon Hotel in Paris on November 15.
Mounted police officers patrol in front of the Eiffel Tower in Paris on November 15.
Security personnel inspect vehicles at the border between Belgium and France on Saturday, November 14.
A police officer stands guard outside the Cafe Bonne Biere in the Rue de la Fontaine au Roi in Paris on November 14.
A French police officer guards the French-Italian border on November 14 in Menton, France.
Soldiers from the 3rd Marine Infantry Parachute Regiment of Carcassonne arrive at Charles de Gaulle Airport in Paris as security reinforcements on November 14.
Soldiers patrol the Eiffel Tower on November 14.
French police check vehicles on the bridge between Strasbourg and Kehl, Germany, on November 14.
Police vehicles are parked near the entrance to the Louvre in Paris on November 14. Interior Minister Bernard Cazeneuve said the state of emergency in France could mean restrictions on people's movements. Airports in France remained open, and airlines were still flying there, though some airlines reported canceled flights.
French police check vehicles on the so-called European bridge between Strasbourg and Kehl, Germany, on November 14.
Police stand guard at a train station in Paris on November 14.
Third, many have familial ties to jihadism. More than a quarter of Western fighters have a familial connection to jihad, whether through relatives who are also fighting in Syria and Iraq, through marriage or through some link to other jihads or terrorist attacks.
How ISIS threatens the West
For instance the father of British ISIS recruit Abdel-Majed Abdel Bary is Adel Abdel Bary, who was convicted in New York for his role in the 1998 U.S. embassy bombings in Kenya and Tanzania.
Of those with a familial link, one third are through marriage, many of them marriages between female recruits and male fighters conducted after they arrive in Syria
Three-fifths of Western fighters with familial ties to jihad have a relative who has also left for Syria. For example, the Deghayes family in the United Kingdom had three sons, ages 16 to 20, leave to join al Nusra Front in Syria together.
Fourth, the Americans drawn to the Syrian jihad -- 250 who have tried or have succeeded in getting to Syria -- share the same profile as the Western fighters overall: Women are well-represented, and the volunteers are young, and many have family ties to jihad. One in six of the Americans drawn to the Syrian conflict are women. The average age of the American militants is 25, with a fifth still in their teens. Almost a fifth of the American militants have a familial connection to jihad.
The American recruits are, perhaps unsurprisingly, particularly active online: Around nine out of 10 American militants are active in online jihadist circles.
Who are ISIS' American recruits?
Fifth, for Western militants, the wars engulfing Syria and Iraq have often proved deadly. Almost half of the male fighters and 6% of the female recruits have been killed in Syria or Iraq.
Opinion: Why Americans leave for Jihad
Sixth, few of the Western fighters who have traveled to Syria and Iraq are in government custody. Only one-fifth of Western fighters in New America's data set are in custody, and more than two-fifths of individuals are still at large. (As indicated above, around half the Western militants were killed in the conflicts in Syria or Iraq.)
Seventh, the most popular route to Syria is through Turkey. Almost half of the Western foreign fighters made their way to Syria or Iraq via Turkey. Only one of the militants is documented as attempting to use an alternative route via Lebanon. For the rest of the Western militants, it's not clear from the public record how they arrived in Syria.
Eighth, where an affiliation can be determined, the majority of the Western fighters have joined ISIS: Three-fifths have joined ISIS, while only a tenth have joined al Qaeda's affiliate in Syria, known as al Nusra Front, and one-seventh have joined other smaller militant groups.
Who is inspiring these militants to give up their often-comfortable lives in the West for the rigors of the war zone in Syria? Based on court records and press reports, New America has identified several Western militants acting as online recruiters. Among them are a number of Americans, for instance:
• Abdi Nur, a 20-year-old from Minnesota, allegedly took on the role of online recruiter after leaving for Syria in the summer of 2014. A complaint filed in November that charged six Minnesota men with trying to go to join ISIS accuses Nur of acting as an online recruiter and providing encouragement and advice to the men via Kik and other social media platforms from Syria.
• Hoda Muthana, a 20-year old American woman from Alabama, was identified by BuzzFeed as the individual behind the Twitter account Umm Jihad, which encouraged militants to leave for Syria.
Propaganda
ISIS has disseminated two online guidebooks to encourage its Western recruits. In 2015, ISIS published its how-to guides Hijrah and "How to Survive in the West." Hijrah provided potential fighters with detailed packing lists -- advice on how to get to Turkey and dupe customs officials into issuing visas for the country; Twitter accounts of fighters living in Syria who can facilitate their travel; and even suggestions for recruits to assess their personality strengths and weaknesses before leaving home to prepare themselves better for jihad.
"How to Survive in the West" is a guide on how to "be a secret agent" in a Western country, giving readers tips on the making of Molotov cocktails, bombs and cell phone detonators; hiding weapons in secret compartments of vehicles, in the same fashion as gangs; and how to identify and evade police surveillance, even suggesting that readers watch the Jason Bourne film series for tips on employing evasion tactics.
In September 2014, an ISIS spokesman called for violence, specifically in France, Australia and Canada, releasing an audiotape saying, "If you can kill a disbelieving American or European -- especially the spiteful and filthy French -- or an Australian, or a Canadian, or any other disbeliever from the disbelievers waging war, including the citizens of the countries that entered into a coalition against the Islamic State, then rely upon Allah, and kill him in any manner or way, however it may be."
Motivations
What motivates many of these Western fighters to travel to a dangerous war zone with which most have no prior connection? A review of both ISIS propaganda and reporting on the individual cases in New America's data set suggests the answer is a mishmash of motivations that ISIS has picked up on as part of its recruiting strategy, including opposition to Syrian dictator Bashar al-Assad, religious invocations of the spiritual benefit of participating in jihad, the belief that religious duty requires living under ISIS' so-called caliphate, anger and alienation from Western society, and for some the "cool" factor of participating in a war.
Here are the rationales for joining ISIS that are provided by a couple of ISIS' alleged American recruits:
• Abdi Nur, the 20-year-old Minnesotan, tweeted: "Jihad Is The Greatest Honor For Man So Come On And Join Dawla Ya Iqwa (you brothers of the Islamic State])." Nur later explained to his sister: "if I didn't care I wouldn't have left but I want jannah (paradise) for all of us."
• Authorities say Chicago teen Hamzah Khan left a letter for his parents before attempting to travel to Syria in 2014, explaining that "there is an obligation to 'migrate' to the 'Islamic State." He was charged with material support of ISIS and has pleaded not guilty.
Threat to United States
Four years into the Syrian civil war, little evidence has emerged to support the notion that returning fighters from Syria pose a great threat to the United States.
In photos: Syria's civil war
Displaced Syrian residents wait to receive food aid distributed by the UN Relief and Works Agency at the besieged al-Yarmouk camp, south of Damascus, Syria, on January 31, 2014. According to the UN Envoy for Syria, an estimated 400,000 Syrians have been killed since an uprising in March 2011 spiraled into civil war. See how the conflict has unfolded.
An injured man lying in the back of a vehicle is rushed to a hospital in Daraa, Syria, on March 23, 2011. Violence flared in Daraa after a group of teens and children were arrested for writing political graffiti. Dozens of people were killed when security forces cracked down on demonstrations.
Anti-government protesters demonstrate in Daraa on March 23, 2011. In response to continuing protests, the Syrian government announced several plans to appease citizens.
Syrian children walk over bricks stored for road repairs during a spontaneous protest June 15, 2011, at a refugee camp near the Syrian border in Yayladagi, Turkey.
Jamal al-Wadi speaks in Istanbul on September 15, 2011, after an alignment of Syrian opposition leaders announced the creation of a Syrian National Council -- their bid to present a united front against Bashar al-Assad's regime and establish a democratic system.
Delegates from Arab League member states and Turkey discuss a response to the government's crackdown in Syria on November 16, 2011.
Supporters of al-Assad celebrate during a referendum vote in Damascus on February 26, 2012. Opposition activists reported at least 55 deaths across the country as Syrians headed to the polls. Analysts and protesters widely described the constitutional referendum as a farce. "Essentially, what (al-Assad's) done here is put a piece of paper that he controls to a vote that he controls so that he can try and maintain control," a US State Department spokeswoman said.
Syrian refugees walk across a field in Syria before crossing into Turkey on March 14, 2012.
Rebel fighters with the Free Syrian Army capture a police officer in Aleppo, Syria, who they believed to be pro-regime militiaman on July 31, 2012. Dozens of officers were reportedly killed as rebels seized police stations in the city.
A Free Syrian Army fighter runs for cover as a Syrian Army tank shell hits a building across the street during clashes in the Salaheddine neighborhood of central Aleppo on August 17, 2012.
Family members mourn the deaths of their relatives in front of a field hospital in Aleppo on August 21, 2012.
A Syrian man carrying grocery bags dodges sniper fire in Aleppo as he runs through an alley near a checkpoint manned by the Free Syrian Army on September 14, 2012.
Free Syrian Army fighters are reflected in a mirror they use to see a Syrian Army post only 50 meters away in Aleppo on September 16, 2012.
Smoke rises over the streets after a mortar bomb from Syria landed in the Turkish border village of Akcakale on October 3, 2012. Five people were killed. In response, Turkey fired on Syrian targets and its parliament authorized a resolution giving the government permission to deploy soldiers to foreign countries.
A Syrian rebel walks inside a burnt section of the Umayyad Mosque in Aleppo hours before the Syrian army retook control of the complex on October 14, 2012.
An Israeli tank crew sits on the Golan Heights overlooking the Syrian village of Breqa on November 6, 2012. Israel fired warning shots toward Syria after a mortar shell hit an Israeli military post. It was the first time Israel fired on Syria across the Golan Heights since the 1973 Yom Kippur War.
Smoke rises in the Hanano and Bustan al-Basha districts in Aleppo as fighting continues through the night on December 1, 2012.
The bodies of three children are laid out for identification by family members at a makeshift hospital in Aleppo on December 2, 2012. The children were allegedly killed in a mortar shell attack that landed close to a bakery in the city.
A father reacts after the deaths of two of his children in Aleppo on January 3, 2013.
Syrians look for survivors amid the rubble of a building targeted by a missile in the al-Mashhad neighborhood of Aleppo on January 7, 2013.
Rebels launch a missile near the Abu Baker brigade in Al-Bab, Syria, on January 16, 2013.
An aerial view shows the Zaatari refugee camp near the Jordanian city of Mafraq on July 18, 2013.
The UN Security Council passes a resolution September 27, 2013, requiring Syria to eliminate its arsenal of chemical weapons. Al-Assad said he would abide by the resolution.
Residents run from a fire at a gasoline and oil shop in Aleppo's Bustan Al-Qasr neighborhood on October 20, 2013. Witnesses said the fire was caused by a bullet from a pro-government sniper.
Syrian children wait as doctors perform medical checkups at a refugee center in Sofia, Bulgaria, on October 26, 2013.
An injured man is helped following an airstrike in Aleppo's Maadi neighborhood on December 17, 2013.
A man holds a baby who was rescued from rubble after an airstrike in Aleppo on February 14, 2014.
A US ship staff member wears personal protective equipment at a naval airbase in Rota, Spain, on April 10, 2014. A former container vessel was fitted out with at least $10 million of gear to let it take on about 560 metric tons of Syria's most dangerous chemical agents and sail them out to sea, officials said.
A Free Syrian Army fighter fires a rocket-propelled grenade during heavy clashes in Aleppo on April 27, 2014.
A giant poster of al-Assad is seen in Damascus on May 31, 2014, ahead of the country's presidential elections. He received 88.7% of the vote in the country's first election after the civil war broke out.
Rebel fighters execute two men on July 25, 2014, in Binnish, Syria. The men were reportedly charged by an Islamic religious court with detonating several car bombs.
Photographs of victims of the Assad regime are displayed as a Syrian army defector known as "Caesar," center, appears in disguise to speak before the House Foreign Affairs Committee in Washington. The July 31, 2014, briefing was called "Assad's Killing Machine Exposed: Implications for U.S. Policy." Caesar, apparently a witness to the regime's brutality, smuggled more than 50,000 photographs depicting the torture and execution of more than 10,000 dissidents. CNN cannot independently confirm the authenticity of the photos, documents and testimony referenced in the report.
Volunteers remove a dead body from under debris after shelling in Aleppo on August 29, 2014. According to the Syrian Civil Defense, barrel bombs are now the greatest killer of civilians in many parts of Syria. The White Helmets are a humanitarian organization that tries to save lives and offer relief.
Medics tend to a man's injuries at a field hospital in Douma after airstrikes on September 20, 2014.
A long-exposure photograph shows a rocket being launched in Aleppo on October 5, 2014.
Rebel fighters dig caves in the mountains for bomb shelters in the northern countryside of Hama on March 9, 2015.
Nusra Front fighters inspect a helicopter belonging to pro-government forces after it crashed in the rebel-held Idlib countryside on March 22, 2015.
A Syrian child fleeing the war gets lifted over fences to enter Turkish territory illegally near a border crossing at Akcakale, Turkey, on June 14, 2015.
A refugee carries mattresses as he re-enters Syria from Turkey on June 22, 2015, after Kurdish People's Protection Units regained control of the area around Tal Abyad, Syria, from ISIS.
A sandstorm blows over damaged buildings in the rebel-held area of Douma, east of Damascus, on September 7, 2015.
Members of a Syrian opposition group attack the headquarters of al-Assad regime forces in the Aleppo villages of Nubul and al-Zahraa on February 12, 2016.
This still image, taken from a video posted by the Aleppo Media Center, shows a young boy in an ambulance after an airstrike in Aleppo, Syria, on August 17, 2016. It took nearly an hour to dig the boy, identified as Omran Daqneesh, out from the rubble, an activist told CNN. The airstrike destroyed his home, where he lived with his parents and two siblings. Director of the Aleppo Media Center Yousef Saddiq said Omran's 10-year-old brother, Ali, died from his injuries.
Syrian men carrying babies make their way through the rubble of destroyed buildings following a reported air strike on the rebel-held Salihin neighborhood of the northern city of Aleppo, on September 11, 2016.
Smoke rises after an airstrike in Aleppo on October 4, 2016.
Arabic writing that reads "some day we will return" is seen on a bus window as civilians evacuate Aleppo on December 15, 2016. The evacuations began under a new ceasefire between rebels and pro-government forces.
This photo, provided by the activist Idlib Media Center, shows dead children after a
suspected chemical attack in the rebel-held city of Khan Sheikhoun on April 4, 2017. Dozens of people were killed, according to multiple activist groups. The United States responded a few days later by launching between 50-60 Tomahawk missiles at a Syrian government airbase. US officials said the base was home to warplanes that carried out the chemical attack. Syria has repeatedly denied it had anything to do with the attack.
Members of the UN Security Council raise their hands on April 12, 2017, as they vote in favor of a draft resolution that condemned the reported use of chemical weapons in Syria.
Residents of the war-torn city of Douma break their Ramadan fast on June 18, 2017.
A member of the Syrian pro-regime forces fires a machine gun as a comrade holds his feeding ammunition belt on November 11, 2017. It was during an advance toward rebel-held positions west of Aleppo.
A child receives medical treatment after a village was attacked in the rebel-held Eastern Ghouta region on February 25, 2018. Several people were treated for exposure to chlorine gas, opposition groups said, as airstrikes and artillery fire from the regime continued. CNN was unable to independently verify claims that chlorine was used as a weapon.
Bodies lie on the ground in the rebel-held city of Douma, Syria, on April 8, 2018.
According to activist groups, helicopters dropped barrel bombs filled with toxic gas on Douma, which has been the focus of a renewed government offensive that launched in mid-February. The Syrian government and its key ally, Russia, vehemently denied involvement and accused rebel groups of fabricating the attack to hinder the army's advances and provoke international military intervention.
Damascus skies erupt with anti-aircraft fire as the US and its allies launch an attack on Syria's capital early on April 14, 2018. US President Donald Trump announced airstrikes in retaliation for Syria's alleged use of chemical weapons. Trump says the strikes are part of a sustained military response, in coordination with France and the United Kingdom.
Women and children who fled the Islamic State group's embattled holdout of Baghouz wait in the back of a truck in the eastern Syrian province of Deir Ezzor, on February 14, 2019.
Members of the Syrian Civil Defence, also known as the "White Helmets," search the rubble of a collapsed building following an explosion in the town of Jisr al-Shughur on April 24, 2019.
Turkish-backed Syrian fighters take part in a battle in Syria's northeastern town of Ras al-Ain as Turkey and its allies continue their assault on Kurdish-held border towns in northeastern Syria on October 14, 2019.
A member of the Syrian Violet Organization carries an injured boy at a makeshift hospital following a regime air strike on a vegetable market in Syria's last major opposition bastion of Idlib on January 15, 2020.
A Syrian man mourns the death of family members next to the rubble of his house following a reported airstrike on the village of Kafr Nuran, in the northern province of Aleppo, on January 21, 2020.
A displaced boy carries a bag of bread as people queue for food distributed by a nonprofit in Idlib on February 19, 2020.
Members of Syrian Civil Defence and Turkish-backed Syrian forces search for victims in the aftermath of a car bomb explosion near a security checkpoint in the town of Azaz on March 19, 2020.
Health workers carry the body of a 62-year-old displaced Syrian man who died of Covid-19, to be buried in the town of Salqin, in Idlib province, on September 17, 2020.
An aerial picture shows Syrians waving the national flag during a gathering in the rebel-held city of Idlib on March 15, 2021, as they mark
10 years since the nationwide anti-government protests that sparked the country's devastating civil war.
In the United States, there has only been one case of a fighter returning from Syria and allegedly plotting an attack. Abdirahman Sheik Mohamud, 22, of Columbus, Ohio, left for Syria in April 2014 and fought there before returning home around two months later. The government alleges that a cleric in Syria told Mohamud that he should return to the United States to conduct an act of terrorism and that he discussed some kind of plan (with an informant) to kill American soldiers at a military base in Texas. He has pleaded not guilty to a charge of providing material support to a terrorist group.
Speaking at the Council on Foreign Relations in March, Director of National Intelligence James Clapper said that about 40 individuals had returned from Syria. "We have since found they went for humanitarian purposes or some other reason that don't relate to plotting," he said.
We identified 23 Americans who actually reached Syria, 46 individuals who attempted or plotted to travel to Syria but were unsuccessful in doing so, and 14 who provided support to others fighting or seeking to fight in Syria.
Instead of being a launch pad for attacks at home, Syria turned out to be a graveyard for the few Americans who made it to the war zone.
Of the 23 individuals who reached Syria, nine died there. For instance, Floridian Moner Abu Salha, died conducting a suicide bombing in northern Syria last year, and Douglas McAuthur McCain was killed fighting for ISIS
Nine of the Americans who reached Syria remain at large, while five American fighters who returned to the United States from Syria were taken into custody.
Threat to United States by non-American ISIS recruits
Many fighters from countries other than the United States have traveled to fight in Syria and could pose a potential threat to the United States. So far we have not seen a case of a foreign fighter from another country traveling to the United States to conduct an attack. However, it is not beyond the realm of possibility.
The large number of foreign fighters traveling to fight in Syria from other countries magnifies the potential threat of an infiltration attack, especially given the high numbers of foreign fighters from countries that enjoy the Visa Waiver Program with the United States, such as Australia, Belgium, France, Germany, the Netherlands and the United Kingdom.
Tracking the many foreign fighters from Western countries who have gone to Syria and who have returned to the West poses a greater challenge, given their larger numbers, than tracking the handful of returning American fighters.
With the large numbers of Europeans traveling to fight in Syria, nations such as France and Germany are reporting significant strain on their ability to monitor returnees effectively.
In December, German's federal prosecutor general, Harald Range, said of the number of terrorism cases being prosecuted in his country, "We are at the limits of our capacity," adding that new cases kept emerging. "What worries me is the speed with which people are radicalizing, or being radicalized. We are facing a phenomenon which needs a broad strategy of prevention."
Each French individual placed under surveillance requires 25 agents to maintain round-the-clock monitoring, and the strain on resources produced by ever increasing numbers of militants who need to be monitored was in part behind the failure to maintain surveillance of the Kouachi brothers, who conducted the attack on the Charlie Hebdo magazine in Paris this year.
It would take many thousands of agents to monitor each of the more than a 1,000 Frenchmen reportedly involved in the Syrian war, and France simply doesn't have that kind of manpower.
The fact that a French prosecutor says that one of the Paris attackers was a French national who was known to police is an indicator of how difficult tracking all of these militants has proven to be.
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