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