(CNN) Mohammad al-Adnani, the official spokesman of ISIS and one of its most senior members, has died in Syria, the terror group said in a rare public statement.
His death marks the highest-profile killing yet of an ISIS member. A key deputy to ISIS leader Abu Bakr al-Baghdadi, he was the person floated to be his successor should anything happen to al-Baghdadi.
A statement from ISIS' Amaq news agency on Tuesday said al-Adnani died while inspecting military operations in the area of Aleppo, Syria. ISIS has not revealed his cause of death and said it "determined to seek revenge" for the killing.
"After a journey filled with sacrifice and fight against non-believers, the Syrian Gallant knight, Abu Mohammed al-Adnani, joined the convoy of martyr leaders," ISIS said.
"To the filthy and coward nonbelievers and to the holders of the Christ emblem, we bring the good news, which will keep them awake, that a new generation in the Islamic State ... that loves death more than life ... this generation will only grow steadfast on the path to Jihad, stay determined to seek revenge and be violent toward them."
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'Significant blow'
Coalition forces have not confirmed his death but the Pentagon acknowledged that he was targeted in a precision strike on Tuesday near Al-Bab, Syria.
"We are still assessing the results of the strike, but al-Adnani's removal from the battlefield would mark another significant blow to ISIS," Pentagon press secretary Peter Cook said in a statement.
"Al-Adnani has served as principal architect of ISIS' external operations and as ISIS' chief spokesman. He has coordinated the movement of ISIS fighters, directly encouraged lone-wolf attacks on civilians and members of the military and actively recruited new ISIS members. The U.S. military will continue to prioritize and relentlessly target ISIS leaders and external plotters in order to defend our homeland, our allies and our partners, while we continue to gather momentum in destroying ISIL's parent tumor in Iraq and Syria and combat its metastases around the world."
More than a spokesman
As spokesman, al-Adnani was the group's most prominent face, the first to announce the ISIS caliphate even before Baghdadi did.
But he was much more than a spokesman, CNN terrorism analyst Paul Cruickshank said.
Al-Adnani was well known for ordering operatives to attack countries participating in the US-led coalition against ISIS. Western intelligence believes he had command responsibility for the Paris attacks.
He is believed to have carried out those orders as one of the ISIS leaders overseeing its security service, known as "Amniyat," which includes the external operations unit tasked with international attacks on the West. As such, the external operations unit reported up to al-Adnani.
A number of ISIS followers and members captured and questioned in the last two years, including French jihadists Faiz Bouchrane and Reda Hame, have attested to al-Adnani as head of the Amniyat.
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"In some ways he's a more dangerous figure than Abu Bakr al-Baghdadi, because he's believed to be overseeing the external operations division of ISIS, and that's the part of ISIS which threatens the West, which carried out the attacks in Paris, which could one day carry out an attack in the US on a significant scale," Cruickshank said.
First to declare ISIS caliphate
Al-Adnani has been with ISIS since its beginnings and was close to al-Baghdadi, CNN's Nic Roberston said. In June 2014, he was the first to declare a "caliphate" for parts of Syria and Iraq, indicating ISIS' aim of not just being a terrorist group but a governing entity.
Born in Syria in 1977, al-Adnani was the most senior Syrian in ISIS, a trait that some analysts say may have counted against him in a leadership bid among the Iraqi-led terror group.
The US State Department officially labeled al-Adnani a terrorist in August 2014 and put a $5 million bounty on his head. They described him as "the official spokesman for and a senior leader" of ISIS, a position he obtained after becoming one of the first foreign fighters to oppose U.S.-led coalition forces in Iraq.
He was arrested in May 2005 in Al-Anbar province and is believed to have spent some time between 2005 and 2010 at the U.S. detention facility, Camp Bucca.
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After his arrival in Syria, he was appointed deputy of Abu Mohammed al-Jawlani, leader of the Islamic State of Iraq for the northern province, then as the security leader before assuming a key role in external operations for ISIS, according to the United Nations Security Council.
Crush their heads with rocks
"He was the mouthpiece of ISIS. He said things like, 'If you can't shoot them, then stab them, and if you can't stab them, then crush their heads with rocks. If you can't do that then drive your cars, your vehicles, to kill them," Robertson said.
"He absolutely tried to maximize every opportunity to instill fear in Syria and Iraq and the international community and send fighters overseas to attack," Robertson said.
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.
In May 2015, the State Department announced a reward of up to $5 million for information leading to al-Adnani, noting his "repeated calls for attacks against Westerns" and his pledges to "defeat" the United States.
He said ISIS supporters in the West had a religious duty to launch lone-wolf attacks, a move analysts call a game changer that may have inspired attacks in North America, Europe and Australia.
"He was the strategic leader of the organization, especially when it comes to attacks on the West," said journalist Graeme Wood, Edward R. Murrow Press Fellow at the Council on Foreign Relations. "To have that voice destroyed is a serious blow to the organization, probably the most significant kill that the enemies of the Islamic state have perpetrated since its declaration of the caliphate."
CNN's Holly Yan and Dugald McConnell contributed to this report.