Two people were "incinerated" in the U.S. airstrike targeting an ISIS terrorist known as "Jihadi John," and a military spokesman said Friday officials are "reasonably certain" he was one of them.
"It will take time to definitively declare whether we had success," Col. Steve Warren, spokesman for the U.S. military operation against ISIS, told reporters during a briefing. "It takes time to confirm."
The U.S. has video showing an individual believed to be Mohammed Emwazi exiting a building in the ISIS stronghold of Raqqa in Syria and getting into a vehicle, a senior defense official earlier told NBC News. A U.S.-operated MQ-9 Reaper drone, one of three that had been tracking Emwazi, then struck the car with two Hellfire missiles, a senior military official also said.
Warren could not comment on the existence of the video, but said the presumed death of Emwazi, a Kuwait-born British citizen, is a "significant blow" to ISIS' image.
"This guy was a human animal. Killing him is probably making the world a little bit better place," Warren said.
He added that a person seen with Emwazi — possibly a driver he characterized as his "best friend" — was also killed, although he was not high level.
Two defense officials say that the U.S. spent "a few days" watching Emwazi before the strike. During that time, he visited some family he has in Raqqa.
When military officials were assured only the intended targets were in the area, "we took the shot," he added.
Both officials stressed how the U.S. has worked closely with the British for the past year to find him. Despite the fact he was not a tactical leader, he was a high priority for both nations, the officials say.
A masked Emwazi had participated in numerous propaganda videos in the killings of Westerners, including American journalists Steven Sotloff and James Foley, American aid worker Abdul-Rahman Kassig, British aid workers David Haines andAlan Henning and Japanese journalist Kenji Goto, officials said.
Speaking earlier, British Prime Minister David Cameron thanked the U.S. for the strike, describing it as "an act of self-defense" and the "right thing to do."
Emwazi, who was 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."
Emwazi — a computer science graduate who lived in a West London neighborhood that 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.
Emwazi's suspected death showed that the broader anti-ISIS strategy was showing signs of succeeding, according to Michael Stephens, a research fellow at the London-based Royal United Services Institute think tank.
"We're chipping away," he said. "There's lots of work being done to target ISIS," Stephens added, referring the wider strategy that included the push by Kurdish forces to retake Sinjar in Iraq.
Simon Palombi, a security consultant at London's Chatham House think tank, was more cautious.
"The only thing that's going to strike fear into the hearts of ISIS fighters is if the Western governments actually commit to fighting them — and they have not done that," he said. "Airstrikes are not a long term strategy."
This article first appeared on NBCNews.com.