LONDON -- Prime Minister David Cameron said Friday that Britain "can't yet be certain" an airstrike in Syria had killed the British man known as "Jihadi John,"who participated in the beheading videos of two American journalists and the slayings of several other captives, according to the Pentagon.
But a U.S. official told CBS News Friday that while they couldn't completely confirm the death of Mohamed Emwazi, they were "pretty goddamn certain" he was killed as he got into a vehicle in the Syrian city of Raqqa. "They're really sure," the official told CBS News national security correspondent David Martin.
On Friday morning, U.S. military spokesman Steve Warren said officials were "reasonably certain" they have killed the militant with the Islamic State of Iraq and Syria, or ISIS, with a Hellfire missile fired from a drone.
Warren said the world was better off without Emwazi, 27, whom the spokesman referred to as a human animal.
Martin reports the drone had been tracking the target believed to be Emwazi since Wednesday, waiting for a clear shot. The missile was fired late Thursday in Syria.
Pentagon press secretary Peter Cook said Thursday that Emwazi was the intended target of the U.S. strike in Raqqa, ISIS' self-proclaimed capital.
Cameron said if proven, Emwazi's death would be a significant blow to ISIS.
He said British officials had "been working with the U.S. around the clock to track him down. This was a combined effort."
Calling the strike an "act of self defense," Cameron described Emwazi as "a barbaric murderer" who posed a threat to people in Syria, "and those around the world."
"I want to thank the U.S... thank those in the intelligence agencies," said Cameron.
The U.K.-based Syrian Observatory for Human Rights, which relies on an extensive network of opposition activists and fighters on the ground in Syria for information, said earlier Friday that all signs indicated Emwazi had been killed.
Local activists reported multiple explosions in Raqqa near an ISIS "court" Thursday night, targeting a vehicle, where they believe he was killed.
Emwazi has been described by a former hostage as a bloodthirsty psychopath who enjoyed threatening Western hostages. Spanish journalist Javier Espinosa, who had been held in Syria for more than six months after his abduction in September 2013, said Emwazi would explain precisely how the militants would carry out a beheading.
Emwazi is in the videos showing the killings of journalists Steven Sotloff and James Foley, U.S. aid worker Abdul-Rahman Kassig, British aid workers David Haines and Alan Henning, Japanese journalist Kenji Goto and a number of other ISIS hostages.
Foley's parents, John and Diane Foley, said Friday that learning of the strike was "a very small solace."
"His death does not bring Jim back," the Foleys said in a statement. "If only so much effort had been given to finding and rescuing Jim and the other hostages who were subsequently murdered by ISIS, they might be alive today."
Sotloff's parents, Art and Shirley Sotloff, said Friday's developments "doesn't change anything for us.
"It's too little too late," the Sotloffs said in a statement. "Our son is never coming back."
Emwazi's identity was confirmed in February by U.S. intelligence officials. Emwazi grew up in West London and graduated in 2009 from the University of Westminster with a degree in computer science.
Officials said Britain's intelligence community had Emwazi on its list of potential terror suspects for years but was unable to prevent him from traveling to Syria.
He had been known to the nation's intelligence services since at least 2009, when he was connected with investigations into terrorism in Somalia.
© 2015 CBS Interactive Inc. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press contributed to this report.