(CNN) -- "We are close" to identifying the ISIS militant who beheaded American journalist James Foley, according to the British ambassador to the United States, Peter Westmacott.
Westmacott told CNN's Candy Crowley on Sunday he couldn't elaborate on the identity of the killer, who is seen decapitating Foley in a video posted last week on YouTube.
"We're putting a great deal into the search," he said, referring to the use of sophisticated technology to analyze the man's voice.
In the video, Foley, 40, is seen kneeling next to a man dressed in black.
Foley reads a message, presumably scripted by his captors, saying that his "real killer'' is America. "I wish I had more time. I wish I could have the hope for freedom to see my family once again," he can be heard saying.
The killer also speaks and experts have said he has a distinctly English accent.
Linguists said that based on his voice, the man appears to be young, most likely under 30. He also appears to have been educated in England from a young age and to be from southern England or London.
The video shows another U.S. journalist, identified as Steven Sotloff, and an ISIS warning of more dead captives if the U.S. continues its military operations.
Foley disappeared on November 22, 2012, in northwest Syria, near the border with Turkey. He was reportedly forced into a vehicle by gunmen; he was not heard from again. At the time of his disappearance, he was working as a freelancer for the U.S.-based online news outlet GlobalPost.
This summer, U.S. special operations units were sent into Syria to rescue Foley and other hostages held by Islamist militants, a U.S. official told CNN. Several dozen of the most elite U.S. commandos from units like Delta Force and Navy SEAL Team 6 flew in on helicopters but couldn't find the hostages, including Foley.
His captors recently sent an e-mail to his family threatening his death -- a message Philip Balboni, the CEO of GlobalPost, described as "vitriolic and filled with rage against the United States."
Foley's captors demanded 100 million euros ($132.5 million) in exchange for his release, Balboni told CNN last week.
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