FBI Director James B. Comey said the United States has determined the identity of the Islamist militant who beheaded two American journalists in Syria, but he declined to provide any additional information on the masked operative who spoke in a British accent.

“I believe we have identified him,” Comey said during a briefing with reporters at FBI headquarters in Washington, the first time that a U.S. official has narrowed the investigation of that killer to a specific suspect.

The videotaped beheadings of U.S. journalists James Foley and Steven Sotloff contributed to the Obama administration’s escalation of its campaign against the Islamic State, the militant group that has been the main target of U.S. airstrikes in Syria this week.

Comey also cited the continued threat posed by a separate al- Qaeda cell in Syria known as the Khorasan group, saying it has been one of his principal concerns in recent months and that he believes the group is probably still largely intact even after it was hit with a barrage of U.S. cruise missiles.

“I believe the group still exists,” Comey said, adding that he is “not confident at all” that an alleged Khorasan effort to mount terrorist attacks against Europe and the United States has been thwarted.

Comey’s remarks coincided with separate comments by Iraq’s prime minister, Haider al-Abadi, that Iraqi intelligence had gathered information indicating that the Islamic State was preparing potential attacks against subway systems in the United States or Europe. But Comey and other senior U.S. officials said they were unaware of such a threat.

The Islamic State is an al-Qaeda offshoot that has seized territory in Iraq and Syria. It has also killed a British aid worker and described the sequence of executions as retaliation against the United States and its allies for military operations against the group.

Because of the killer’s distinct accent, the investigation into the killings of Foley and Sotloff have focused on a pool of hundreds of British militants who are believed to have traveled to Syria to fight.

A senior U.S. intelligence official said investigators had ruled out as a suspect a young rapper of Egyptian descent named Adel Abdel Bary, who traveled last year from London to Syria. Bary’s father was extradited to the United States in 2012 to stand trial for his suspected role in the 1998 U.S. embassy bombings in East Africa. He pleaded guilty last week to terrorism charges in New York, but a federal judge expressed concerns about the deal and has yet to accept it.

According to people familiar with the investigation, the British-accented killer spoke excellent Arabic and wore a mask at all times while overseeing the hostages.

The people said he probably grew up in an Arabic-speaking home in England. One person said the killer’s family possibly came from Yemen or Sudan.

Authorities have been using voice analysis to try to determine the killer’s identity, but another U.S. intelligence official said the FBI was able to identify the killer using other means.

Iraqi Prime Minister Haider al-Abadi said he has received "credible" intelligence that Islamic State militants plan to attack subway systems in Paris and the United States. (Reuters)

Adam Goldman contributed to this report.