A U.S. drone strike targeted a vehicle in Syria believed to be transporting the masked Islamic State militant known as “Jihadi John,” U.S. officials said, but it was still unclear whether the strike killed the British man who appears in several videos depicting the beheadings of Western hostages.

Mohammed Emwazi, a Kuwaiti-born British citizen, was the target of an airstrike in Raqqa, Pentagon press secretary Peter Cook said in a statement. Officials were assessing the results of the strike, he said.

U.S. military spokesman Steve Warren said officials were “reasonably certain” they had killed Jihadi John with a Hellfire missile fired from a drone. Another U.S. official told The Associated Press that a drone had targeted a vehicle in which Emwazi was believed to be traveling. The official was not authorized to speak publicly and requested anonymity.

Warren said the world is better off without the man believed to have beheaded several Western hostages, whom he referred to as a human animal. He said the operation was one in a string of targeted attacks on Islamic State leaders. He says the U.S. has killed one mid- to upper-level ISIL leader every two days since May.

British Prime Minister David Cameron said the strike had been a joint effort and that British intelligence agencies were working around the clock to find the British-accented militant, whom Cameron called the militant group’s “lead executioner.”

Cameron also said the U.S. strike had been “an act of self-defense” and the right thing to do. He said targeting Emwazi was “a strike at the heart” of the Islamic State group.

U.S. Secretary of State John Kerry, appearing at a news conference in Tunis, Tunisia, on Friday told reporters extremists “need to know this: Your days are numbered, and you will be defeated.”

British Foreign Secretary Philip Hammond, in Prague, said officials were “obviously pursuing all avenues to confirm that he is dead although we believe the strike was successful.”

The Britain-based Syrian Observatory for Human Rights said that U.S. coalition warplanes struck an Islamic State vehicle as it left the governor’s office in the group’s self-proclaimed capital of Raqqa, Syria, killing four IS foreign fighters, including a British commander. The Observatory said the bodies were charred, and Observatory chief Rami Abdurrahman said the commander killed in the attack was most likely Jihadi John but that he does not have 100 percent confirmation.

If a drone strike did indeed kill Emwazi, it would represent the latest in a string of significant Islamic State and Khorasan Group figures who have been tracked and killed in a joint effort by the CIA and the military’s Joint Special Operations Command.

In an effort that ramped up over the last year, intelligence analysts and special operators have harnessed an array of satellites, sensors, drones and other technology to track and kill elusive militants across a vast, rugged area of Syria and Iraq, overcoming the lack of a significant U.S. ground presence and the awareness by American targets that they can be found through their use of electronic devices.

The CIA began stepping up efforts to profile militants in Syria in early 2013, even before the Islamic State had seized significant territory. But its tracking capacity has improved as the Pentagon has deployed 24-hour overhead coverage allowing the NSA to soak up electronic signals while the National Geospatial-Intelligence Agency (NGA) conducts visual surveillance, officials say. The CIA and Defense Intelligence Agency have stepped up efforts to recruit human sources.

Emwazi, believed to be in his mid-20s, 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.

Those being held by three British-sounding captors nicknamed them “the Beatles,” with “Jihadi John” a reference to Beatles member John Lennon, Espinosa said.

Among those beheaded by Islamic State militants in videos posted online since August 2014 were U.S. journalists Steven Sotloff and James Foley, U.S. aid worker Abdul-Rahman Kassig, British aid workers David Haines and Alan Henning, and Japanese journalist Kenji Goto.

Their friends and relatives all said Friday that even if Emwazi was dead, it would bring little comfort.

A friend of Henning’s has said she is still “skeptical” following news that “Jihadi John” may have been killed. Foley’s parents, John and Diane Foley, of New Hampshire, issued a statement calling Emwazi’s purported death “a very small solace.”

“His death does not bring Jim back.” ”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,” said the Foleys’ statement, using an alternate acronym for the Islamic State.

In the videos, a tall masked figure clad in black and speaking in a British accent typically began one of the gruesome videos with a political rant and a kneeling hostage before him, then ended it holding an oversize knife in his hand with the headless victim lying before him in the sand.

Emwazi was identified as “Jihadi John” last February, although a lawyer who once represented Emwazi’s father told reporters that there was no evidence supporting the accusation. Experts and others later confirmed the identification.

Emwazi was born in Kuwait and spent part of his childhood in the poor Taima area of Jahra before moving to Britain while still a boy, according to news reports quoting Syrian activists who knew the family. He attended state schools in London, then studied computer science at the University of Westminster before leaving for Syria in 2013. The woman who had been the principal at London’s Quintin Kynaston Academy told the BBC earlier this year that Emwazi had been quiet and “reasonably hard-working.”

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.

The beheading of Foley, 40, of Rochester, New Hampshire, was deemed by IS to be its response to U.S. airstrikes. The release of the video, on Aug. 19, 2014, horrified and outraged the civilized world but was followed the next month by videos showing the beheadings of Sotloff and Haines and, in October, of Henning.

(AP)