The notorious ISIS terrorist known as "Jihadi John" was targeted in a U.S. airstrike but the murderer's fate remained unclear early Friday.
Mohammed Emwazi has participated in numerous propaganda videos showing the killings of Westerners, including American journalists Steven Sotloff and James Foley, American aid worker Abdul-Rahman Kassig, British aid workers David Haines and Alan Henning and Japanese journalist Kenji Goto, officials said.
A senior U.S. official told NBC News that the Kuwait-born British citizen was directly "targeted" by the strike in the ISIS stronghold of Raqqa, Syria, on Thursday night.
"There is no vengeance, but there is accountability," said the official, who stressed that "we are still assessing and not confirming" whether Emwazi had been killed.
A senior U.S. counter-terrorism official also told NBC News early Friday that there was "no definitive proof yet that he was killed."
British Prime Minister David Cameron described the U.S. strike as "an act of self-defense" and the "right thing to do."
But Cameron admitted that "we cannot yet be certain" that the attack had been successful.
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Describing Emwazi as a "barbaric murderer" and the militant group's "lead executioner," he added: "He posed an ongoing and serious threat to innocent civilians ... He was intent on murdering many more people."
Cameron also thanked the U.S. carrying out the strike.
Sotloff's mother, Shirley Sotloff, said she hadn't been informed about the airstrike.
"If they got him great," she told NBC News. But "it doesn't bring my son back."
Sofloff added: "I don't think there will ever be closure."
Related: How Victims' Kin Reacted to News of U.S. Strike
Emwazi, who is in his mid-20s, was identified as the masked knife-wielding man in the gruesome execution videos in February. He was dubbed "Jihadi John" by hostages because he was one of four British terrorists whom their prisoners named "The Beatles."
He is also thought to have used the nom de guerre "Abu Saleh."
Emwazi — a computer science graduate who lived in a west London neighborhood which has produced several other terrorists before leaving for Syria in 2013 — had been known to security services and was detained several times dating to 2009. He was interrogated but was never arrested or charged.
Britain's MI5 intelligence agency believes he was a member of a network of supporters of terrorism and had been in contact with one of the men convicted of attempting to bomb London's transit system on July 21, 2005. The successful suicide attacks killed 52 commuters.
Related: From Would-Be Soccer Star to the Face of ISIS
Charlie Winter, a London-based counter-terrorism analyst, described Jihadi John" as a "key symbol of ISIS menace."
Winter added: "He has done all he can to keep as low a profile as possible and that he has been targeted in an attack is significant ... It points to a really sophisticated level of human intelligence from the U.S. and the coalition."
In March, lawyers for Emwazi's father insisted there was "no evidence" that his son was "Jihadi John."
However, a colleague said the suspect's dad had earlier responded to allegations that Emwazi was an ISIS murderer by saying: "May God ... take revenge on him."
Related: What Will 'Jihadi John' Strike Mean for ISIS?
A former co-worker of the alleged killer also told NBC News that Emwazi was a model employee at a Kuwaiti tech company where he worked as salesman in 2010 after graduating from college.
"He was very calm and polite, well-mannered," the ex-colleague said. "There was nothing at all that made us question him, nothing at all ... He was very committed, disciplined. He never lost his temper or showed any violent tendencies."
