(CNN) The deadliest terrorist attack on U.S. soil since 9/11 followed a clarion call by ISIS to its supporters in the United States to launch attacks during Ramadan, the Islamic Holy Month, which started last week.
In an audio recording released on May 21, ISIS spokesman Abu Mohammed al Adnani called for "a month of hurt" in the United States as well as Europe.
His message to ISIS sympathizers like Orlando shooter Omar Mateen: Stay home and kill anybody, anyhow, anywhere.
'Terrorize them'
"The smallest action you do in the heart of their land is dearer to us than the largest action by us, and more effective and more damaging to them," al Adnani said, according to a translation by the SITE Intelligence Group.
"And if one of you wishes and tries hard to reach the Islamic State, then one of us wishes to be in your place to hurt the Crusaders day and night without sleeping, and terrorize them so that the neighbor fears his neighbor."
It was the latest in a series of calls for attacks by Adnani, whose fatwas describing hitting the West as religious duty have motivated multiple plotters on both sides of the Atlantic to launch attacks.
It was also the most strident call for attacks yet and a sign ISIS is mobilizing its supporters and fighters to wage an all-out campaign of revenge as it loses territory in Syria, Iraq and Libya.
Western intelligence officials believe Adnani oversees ISIS' external attack plotting and had command responsibility for the Paris and Brussels attacks.
Orlando nightclub shooting
Police investigate the back of the Pulse nightclub in Orlando on Sunday, June 12. At least 49 people were killed there by Omar Mateen, who was shot and killed by Orlando police. It was the deadliest mass shooting in U.S. history.
Bodies arrive at the medical examiner's office on June 12.
Friends and family react after a list of hospitalized victims is released June 12 outside a hotel near the Orlando Regional Medical Center.
Pastor Kelvin Cobaris embraces Orlando City Commissioner Patty Sheehan, right, and Terry DeCarlo, executive director of the LGBT Center of Central Florida, on June 12.
Jeremy Glatstein donates blood in Orlando on June 12. He drove an hour to the donation center to show his support for the shooting victims.
A bomb disposal unit checks for explosives around the apartment building where Mateen is believed to have lived in Fort Pierce, Florida.
Ray Rivera, a DJ at the nightclub, is consoled by a friend outside of the Orlando Police Department.
Florida Gov. Rick Scott arrives at the scene.
Police gather outside the home in Port St. Lucie, Florida, where Mateen's father lives.
Police officers gather outside of the nightclub, which describes itself as "the hottest gay bar" in the heart of Orlando.
Ron Hopper of the FBI answers questions from members of the media on June 12. Listening are Orlando Police Chief John Mina, left, and Orange County Mayor Teresa Jacobs.
A police officer stands guard outside the Orlando Regional Medical Center.
A SWAT team member arrives at the scene of the shooting.
Medical personnel wait with stretchers at the Orlando Regional Medical Center.
People wait for news outside the emergency entrance of the hospital.
Police in Orlando direct family members away from the scene of the shooting.
Shooting victims are attended to by emergency responders outside the club.
Emergency personnel gather outside the nightclub.
Bystanders wait at the scene.
Police officers respond to the scene of the shooting.
Loss of appeal?
The Orlando attack comes at a time when there are some signs ISIS is losing its appeal in the United States.
Last month, FBI Director James Comey said there had been a sharp drop in the number of Americans attempting to travel to Syria and Iraq to join ISIS.
In the first half of 2015, the FBI was seeing six to 10 American residents trying to travel each month, a figure that dropped to just one per month since August 2015. But one concern officials have had is that American ISIS sympathizers unable to reach the so-called caliphate would attempt to launch attacks in the United States instead.
In previous messages, ISIS has told its supporters they will be rewarded tenfold in paradise for carrying out attacks during Ramadan, an example of them inventing theological precepts to suit their purposes.
The group has also made clear its visceral and vicious homophobia by throwing people they suspect of being gay off buildings in their so-called caliphate and stoning them.
The ISIS terror threat
People flee the scene of a terror attack at Istanbul's Ataturk airport on June 29. Turkish officials have strong evidence that ISIS leadership was involved in the planning of the attack, a senior government source told CNN. Officials believe the men -- identified by state media as being from Russia, Uzbekistan and Kyrgyzstan -- entered Turkey from the ISIS stronghold of Raqqa in Syria, bringing with them the suicide vests and bombs used in the attack,
the source said.
The ISIS militant group -- led by Abu Bakr al-Baghdadi, pictured -- began as a splinter group of al Qaeda.
Its aim is to create an Islamic state, or caliphate, across Iraq and Syria. It is implementing Sharia law, rooted in eighth-century Islam, to establish a society that mirrors the region's ancient past. It is known for killing dozens of people at a time and carrying out public executions.
Kurdish Peshmerga fighters fire missiles during clashes with ISIS in Jalawla, Iraq, on June 14, 2014. That month, ISIS took control of Mosul and Tikrit, two major cities in northern Iraq.
Traffic from Mosul lines up at a checkpoint in Kalak, Iraq, on June 14, 2014. Thousands of people
fled Mosul after it was overrun by ISIS.
ISIS fighters parade down an Iraqi street in this image released by the group in July 2014.
Aziza Hamid, a 15-year-old Iraqi girl, cries for her father while she and other Yazidi people are flown to safety after a
dramatic rescue operation at Iraq's Mount Sinjar on August 11, 2014. A CNN crew
was on the flight, which took diapers, milk, water and food to the site where as many as 70,000 people were trapped by ISIS. Only a few of them were able to fly back on the helicopter with the Iraqi Air Force and Kurdish Peshmerga fighters.
On August 19, 2014, American journalist James Foley
was decapitated by ISIS militants in a video posted on YouTube. A month later, they released videos showing the executions of American journalist Steven Sotloff and British aid worker David Haines.
ISIS militants stand near the site of an airstrike near the Turkey-Syria border on October 23, 2014. The United States and several Arab nations
began bombing ISIS targets in Syria to take out the group's ability to command, train and resupply its fighters.
A Kurdish marksman stands atop a building as he looks at the destroyed Syrian town of Kobani on January 30, 2015. After four months of fighting, Peshmerga forces
liberated the city from the grip of ISIS.
Safi al-Kasasbeh, right, receives condolences from tribal leaders at his home village near Karak, Jordan, on February 4, 2015. Al-Kasasbeh's son,
Jordanian pilot Moath al-Kasasbeh, was burned alive in a video that was released by ISIS militants. Jordan is one of a handful of Middle Eastern nations taking part in the U.S.-led military coalition against ISIS.
In February 2015, British newspapers report the identity of "Jihadi John," the disguised man with a British accent who had appeared in ISIS videos executing Western hostages. The militant was identified as Mohammed Emwazi, a Kuwaiti-born Londoner. On November 12, 2015, the Pentagon announced that Emwazi was in a vehicle
hit by a drone strike. ISIS later confirmed his death.
In March 2015, ISIS released video and images of a man being thrown off a rooftop in Raqqa, Syria. In the last photograph, the man is seen face down, surrounded by a small crowd of men carrying weapons and rocks. The caption reads "stoned to death." The victim was brutally killed
because he was accused of being gay.
An Iraqi soldier searches for ISIS fighters in Tikrit on March 30, 2015. Iraqi forces
retook the city after it had been in ISIS control since June 2014.
Dead bodies lie near a beachside hotel in Sousse, Tunisia, after
a gunman opened fire on June 26, 2015. ISIS claimed responsibility for the attack, which killed at least 38 people and wounded at least 36 others, many of them Western tourists. Two U.S. officials said they believed the attack might have been inspired by ISIS but not directed by it.
ISIS also claimed responsibility for what it called a suicide bombing
at the Al-Sadiq mosque in Kuwait City on June 26, 2015. At least 27 people were killed and at least 227 were wounded, state media reported at the time. The bombing came on the same day as the attack on the Tunisian beach.
A man inspects the aftermath of a car bombing in Khan Bani Saad, Iraq, on July 18, 2015.
A suicide bomber with an ice truck, promising cheap relief from the scorching summer heat, lured more than 100 people to their deaths. ISIS claimed responsibility on Twitter.
Two women hold hands after an explosion in Suruc, Turkey, on July 20, 2015. The blast
occurred at the Amara Cultural Park, where a group was calling for help to rebuild the Syrian city of Kobani, CNN Turk reported. At least 32 people were killed and at least 100 were wounded in the bombing. Turkish authorities said they believed ISIS was involved in the explosion.
Spectators at the Stade de France in Paris run onto the soccer field after explosions were heard outside the stadium on November 13, 2015. Three teams of gun-wielding ISIS militants
hit six locations around the city, killing at least 129 people and wounding hundreds.
Law enforcement officers search a residential area in San Bernardino, California, after a
mass shooting killed at least 14 people and injured 21 on December 2, 2015.
The shooters -- Syed Rizwan Farook and his wife, Tashfeen Malik -- were fatally shot in a gunbattle with police hours after the initial incident. The couple supported ISIS and had been planning the attack for some time, investigators said.
Two wounded women sit in the airport in Brussels, Belgium, after two explosions rocked the facility on March 22, 2016. A subway station in the city
was also targeted in terrorist attacks that killed at least 30 people and injured hundreds more. Investigators say the suspects belonged to the same ISIS network that was behind the Paris terror attacks in November.
A boy walks past bloodstains and debris at a cafe in Balad, Iraq, that was attacked by ISIS gunmen on May 13, 2016. Twenty people were killed.
Iraqi government forces patrol in southern Falluja, Iraq, on June 10, 2016. In late June,
a senior Iraqi general announced that the battle to reclaim Falluja from ISIS had been won.
No direct ties
Orlando gunman Mateen swore allegiance to ISIS in a 911 call as his attack got underway, providing ISIS with an opportunity to claim ownership of the attack. Hours after the massacre, the ISIS-affiliated Amaq news agency claimed the attack had been carried out by an ISIS fighter, without offering any proof of advance knowledge.
As of Sunday night no evidence had emerged of direct organizational ties between Mateen and ISIS.
The emergence of ISIS has seen its message resonate in the United States over the past two years, partly because it has so skillfully exploited social media to spread its propaganda and create a virtual community of like-minded followers who constantly interact and reinforce each other.
According to a database maintained by the majority staff of the House Homeland Security Committee, U.S. law enforcement agencies have 1,000 active investigations into U.S. homegrown jihadis, 80% of which involved ISIS sympathizers.
Since 2014 there have been 87 sympathizers charged with terror-related offences in the United State and there have been 25 terrorist plots inspired by or instigated by ISIS. Many of those were thwarted in FBI sting operations.
Under the radar
The high number of cases is likely one of the reasons Mateen was not under surveillance at the time of the attacks.
With investigations in 50 states, U.S. law enforcement agencies have had to prioritize. Mateen first came across the FBI radar screen in 2013 after co-workers flagged signs of radicalization.
In 2014, the FBI interviewed Mateen again over possible connections with a fellow Floridian, Moner Mohammad Abu-Salha, who joined al Qaeda in Syria and blew himself up in a suicide truck bombing in May that year.
In a video recorded before his death, Salha stated he had attempted to recruit Florida friends to travel with him to Syria. The FBI found no grounds for continuing its investigation of Mateen.
"We determined that contact was minimal and did not constitute a substantive relationship or threat at that time," said FBI Assistant Special Agent in Charge Ronald Hopper.
Few but deadly
American Muslim leaders were swift to condemn the Orlando shootings. The overwhelming majority of American Muslims reject ISIS' ideology, with radicalization rates being far lower in the United States than in European countries like France or the United Kingdom.
But a key and growing concern of U.S. counterterrorism officials is that ISIS operatives in Syria and Iraq are communicating directly with American sympathizers by using online encryption messaging apps, grooming them for attacks.
On the morning of an attack at a "Draw the Prophet Mohammed Contest" in Garland, Texas, in May 2015, British ISIS operative Junaid Hussain exchanged 109 messages with one of the gunmen.
"In the course of the period from April 1 to July 4 in New York City, and from Boston to Morgantown, North Carolina, we had a dozen arrests in three or four plots, two of which targeted New York City directly, and this was all based on ISIS meeting people on Twitter and talking to them on encrypted apps," NYPD counterterrorism Commissioner John Miller told CTC Sentinel earlier this year.
"What we were seeing was a pace of cases and arrests and plotters that we hadn't seen before. We were seeing that the mass marketing of terrorism was starting to be more effective than we had ever seen with any other kind of messaging before."
Another longstanding concern of U.S. counterterrorism officials is the relatively easy access to powerful weapons in the United States.
There have been a string of deadly Islamist terrorist attacks involving firearms in the United States, including the killing of five members of the U.S. military in July 2015 in Chattanooga, Tennessee, and 14 people at a holiday party last December in San Bernardino, California, an attack also inspired by ISIS.
In 2011 al Qaeda instructed its followers to take advantage of what by international standards are weak gun laws.
While terrorist groups don't have nearly as many supporters in the United States, it's much easier for followers of such groups to get guns.