The United States believes it has identified the ISIS militant in the video showing American journalist James Foley's killing, FBI Director James Comey said Thursday.

Comey did not publicly identify the man.

Since August, ISIS has beheaded Foley, American journalist Steven Sotloff and British aid worker David Haines, gruesomely showing their killings in videos posted online.

FBI officials believe the same militant speaks in all three videos.

In them, he wears a mask and is dressed head-to-toe in black. The militant wields a knife and speaks in a British accent.

U.S. analysts have said it's not possible to know from the videos who, exactly, carried out the killings. The pictures fade to black, and the slayings are not shown in their entirety.

In late August, British Ambassador Peter Westmacott said his country was close to identifying the ISIS militant. Westmacott didn't elaborate about the identity in the August 26 interview, which aired on CNN's "State of the Union" with Candy Crowley.

"We're putting a great deal into the search," Westmacott said at the time, referring to the use of sophisticated technology to analyze the man's voice.

ISIS, which calls itself the Islamic State, has been ramping up its threats against the United States and the West.

This summer, the group declared the establishment of a "caliphate," an Islamic state stretching across the territory it has conquered.

"The reason this is so important," Republican Rep. Ed Royce of California told CNN about the militant's identification, "is that there's close to 1,000 young men like this one from Britain who are fighting with this ISIS organization today, as well as 300 from the United States, approximately. And they hold these passports.

"The fact that they're willing to commit this kind of mayhem and murder -- and at the same time advocate coming back to Europe and coming to the United States and carrying out these attacks -- is the very reason why Interpol, why the British intelligence agencies, and the United States are working right now to track exactly who these young men are," he said.

Royce, who chairs the House Foreign Affairs Committee, said that authorities would be following up on the militant's contacts in Britain.

Foley disappeared on November 22, 2012, in northwest Syria, near the border with Turkey. At the time of his disappearance, he was working for the U.S.-based online news outlet GlobalPost. The video of his beheading was posted to YouTube on August 19.

Sotloff disappeared while reporting from Syria in August 2013. The video of his death was posted September 2.

Haines was abducted in March 2013 near a refugee camp in Atmeh, Syria, where he was working to arrange for the delivery of humanitarian aid to people staying at the camp. A video of his death was posted online September 13.

(CNN)