(CNN) British officials "are close" to identifying the ISIS militant who beheaded American journalist James Foley, according to Britain's 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, who speaks with what experts say is a distinctly English accent.
Linguists said that based on his voice, the man sounds to be younger than 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, being held by ISIS. The militant warned that Sotloff's fate depends on what President Barack Obama does next in Iraq.
A day after the video was posted, Obama vowed the United States will be "relentless" in striking back against ISIS.
Airstrikes continued to hit ISIS targets near Irbil and the Mosul Dam on Sunday, U.S. Central Command said in a news release. The majority of the strikes have been in support of Iraqi forces near the dam, which briefly fell under ISIS control.
Meanwhile Sunday, hundreds of mourners crammed into Foley's hometown church to attend his memorial Mass.
"This moment in our lives is international in scope; crossing all boundaries, yet very personal," Bishop Peter A. Libasci said. "[We are] bound together by a deep sense of human compassion and heartfelt remorse."
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 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.