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.