ISIS’s most famous executioner, Mohammed Emwazi—best known as “Jihadi John”—was targeted in a U.S. airstrike in Syria early Friday morning, according to a senior U.S. administration official.

Emwazi was part of “The Beatles,” three British ISIS members that held, taunted, and tortured Western prisoners. Emwazi is suspected of carrying out numerous executions for the so-called Islamic State, including the beheading of American journalist James Foley and other American hostages.

A U.S. defense official told The Daily Beast that the U.S. military followed Emwazi for the better part of a day leading up to the strike, which happened as he left a building. While officials cannot officially say he is dead—and won’t be able to for some time—they are all but certain.

“We are pretty damn sure he is dead,” the defense official said.

Army Col. Steve Warren told reporters on Friday that officials were “reasonably certain we killed the target that we intended to kill, which is Jihadi John.”

“It was a drone strike using a Hellfire missile,” he said, noting that the strike itself was “fairly routine.” The military has been killing high-level or prominent ISIS fighters like Emwazi at the rate of about one every two days since May, Warren said.

British Prime Minister David Cameron called the drone strike an act of “self defense” against Emwazi, who he called a “barbaric murderer” and an “ongoing and serious threat.” Labour Party leader Jeremy Corbyn said it would’ve been “far better” if Emwazi was tried in court instead of killed.

A senior U.S. official echoed Cameron.

“This isn’t about avenging deaths, but removing a despicable individual who committed brutal murders under the false pretense of a bankrupt and hijacked ideology,” the official told The Daily Beast.

On Thursday night, the U.S. government began notifying the families of the American hostages that Emwazi is thought to have been killed, according to the official. In addition to Foley, Emwazi is believed to have murdered American journalist Steven Sotloff and American aid worker Abdul-Rahman Kassig.

Diane Foley, the mother of journalist James Foley, who was the first American shown murdered by ISIS, said news of the terrorist's possible death was “really a small solace to us,” she told ABC News.

“This huge effort to go after the this deranged man filled with hate when they can’t make half that effort to save the hostages while these young Americans were still alive,” Foley said.

The hostages being held by ISIS gave Emwazi the nickname “Jihadi John,” which is how the world came to know him.

He first appeared in the video showing the killing of Foley in August 2014. Dressed all in black, with a balaclava showing only his eyes, Emwazi spoke in British-accented English that sent shivers down the spine.

After several more beheading videos, The Washington Post revealed the identity of the man behind the mask in February, when his friends told the newspaper that they had no doubt that Emwazi was the man appearing in the ISIS execution videos.

Before launching the airstrike that may have killed Emwazi, the U.S. notified the British government, the senior administration official said.

President Obama said in an interview conducted Thursday with ABC News that the U.S. has “contained” ISIS but wants to “completely decapitate their command and control structures.”

In addition to the American hostages, Emwazi, who was in his mid-twenties, is believed to have murdered British aid workers David Haines and Alan Henning.

Emwazi is also believed to have participated in the killing of a number of other hostages, including Japanese journalist Kenji Goto.

According to the testimony of a former hostage, Emwazi participated in the waterboarding of at least four Western hostages.

Emwazi was born in Kuwait and moved to Britain when he was 6 years old. He grew up in a middle-class neighborhood in West London and graduated with a degree in computer science from the University of Westminster in 2009.

His friends said they thought he’d started to radicalize after a trip he took to Tanzania following his graduation from university. Emwazi was stopped at Dar-es-Salaam by security officials who believed he was not there for a safari. Emwazi was then sent to the Netherlands, where he was interrogated by MI5.

In 2010, British intelligence suspected Emwazi was part of a terrorist network sending fighters to East Africa for al Shabab. Yet he remained in the U.K. and free until 2012, when he was reported missing in Syria.

—with additional reporting by Nancy A. Youssef and Shane Harris.