By DAVID E. SANGERSEPT. 25, 2014

WASHINGTON — American intelligence agencies believe they have identified the Islamic State militant who appeared on two videotapes in which American journalists were beheaded, the F.B.I. director, James B. Comey, said Thursday, but he declined to name the man while agents from the United States and Britain were searching for him.

Intelligence agencies have used voice-recognition technology, overhead imagery and records of Western fighters who are believed to have joined the group in the effort to identify the killer, who first appeared in a video a month ago showing the beheading of James Foley. A second gruesome video, showing the death of Steven J. Sotloff, was released about two weeks later. Both men were freelance journalists.

For a while, British officials focused their suspicions on a rapper who they believed had gone to Syria to fight. Now “the assumption is that was wrong,” one official said.

On Wednesday, the United States announced sanctions against a number of members of the Islamic State, including Salim Benghalem, whom it identified as a fighter “who carries out executions on behalf of the group.” But he was identified as coming from France, suggesting that the two Americans were killed by someone else.

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Mr. Comey also referred to a fighter with “North American-accented English” who was seen on another video titled “Flames of War.” At the end of that video, a masked man, waving a gun, speaks in fluent English as a group of men appear in the background, digging what seems to be a mass grave. The video later appears to show the men being shot from behind.

Mr. Comey also discussed the airstrikes this week in Syria, but was cautious in addressing whether some of the attacks against a second terrorist organization known as the Khorasan Group had disrupted its efforts to carry out attacks in Europe or the United States.

“I believe the group still exists,” Mr. Comey said.

Earlier in the day, Iraq’s new prime minister, Haider al-Abadi, warned that the Islamic State was focused on attacks in the subways in Paris and the United States, but Mr. Comey and the National Security Council said they were not aware of any evidence supporting that charge.

In the Foley and Sotloff videos, the masked killer, armed with a short knife, taunts President Obama and says that the beheadings are in retaliation for bombing attacks on Islamic State targets in Iraq. The killings appear to take place in the same desert location, with hills visible in the background. The executioner speaks in a British accent, which led to a focus on men who hold British passports and are believed to have joined the fight on behalf of the Islamic State, also known as ISIS.

President Obama referred briefly to the beheadings of the two journalists and a third killing, of a British aid worker, during his speech on Wednesday to the United Nations General Assembly. “In the most horrific crimes imaginable, innocent human beings have been beheaded, with videos of the atrocity distributed to shock the conscience of the world,” he said.

Those comments set up what was perhaps the strongest language of the speech, in which Mr. Obama made the case for direct military action: “No God condones this terror. No grievance justifies these actions. There can be no reasoning — no negotiation — with this brand of evil. The only language understood by killers like this is the language of force.”