(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.
Mourners pack Foley's hometown church
Meanwhile Sunday, hundreds of mourners crammed into Foley's hometown church in New Hampshire 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's parents, who received a standing ovation, asked for privacy and thanked mourners for their support.
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.
His family released a letter Sunday said to be written in June by Foley. Because his letters were confiscated in captivity, Foley's family said he asked another hostage set to be released to commit the letter to memory.
In the letter, Foley reflects on favorite family memories -- a trip to the mall with his father, a bike ride with his mom -- and gives details of his time in captivity.
Comfort from others being held
"Eighteen of us have been held together in one cell, which has helped me. We have had each other to have endless long conversations about movies, trivia, sports," Foley wrote, describing makeshift games of checkers, chess and Risk.
"The games and teaching each other have helped the time pass. They have been a huge help."
He had specific messages of love for his brothers and sister, and to his grandmother he told her, "please take your medicine, take walks and keep dancing."
By the time he wrote the letter, he had already been held captive for a year and half, and seemed to waver between remaining hopeful for his release, while also resigned to his fate. While addressing his brothers and sister, he gives specific wishes on who his money should go to and thanks them for "happy childhood memories." But he closes the letter by addressing his "Grammy."
"Stay strong," he told her, "because I am going to need your help to reclaim my life."
The following month, over the July 4 weekend, 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.
CNN's Joshua Berlinger contributed to this report.