American intelligence agencies believe that they have identified the Islamic State militant who appeared in three videos showing the beheading of two American journalists and a British aid worker.

On Thursday, FBI director James B. Comey announced the U.S. progress in tracking down the ISIS fighter in the videos, but he declined to reveal the man's name.

In August, ISIS released a video of the beheading of American journalist James Foley, who was reporting on the civil war in northern Syria in Nov. 2012 when he was kidnapped by the infamous terror group, reports the New York Daily News.

The militants in the video, titled "A Message to #America (from the #IslamicState)," said the beheading was retribution for recent U.S. airstrikes in Iraq.

On Sept. 2, the Islamic State militants released another video, this time showing the beheading of American journalist Steven Sotloff, who freelanced for Time and Foreign Policy magazines before he disappeared in Syria in Aug. 2013. In the video, titled "A second message to America," Sotloff recites a message before he is executed by extreme Iraqi terrorists.

The last beheading video shows the execution of David Haines, who was working at refugee camp in Syria when he was abducted in March 2013. The video was posted online on Sept. 13.

In each video, FBI officials believe that the same militant is speaking in a British accent while wearing a mask and all black.

Intelligence agencies have used a variety of techniques to identify the killer, including voice-recognition technology, distinct topography and records of Western fighters believed to have joined ISIS.

The latest ISIS video, released earlier this week and titled "Flames of War," features another man who speaks with perfect English. At the end of that video, the masked man waves a gun while a group of men appear to be digging a grave in the background. The video then appears to show the men being shot from behind.

"There is no doubt [he] is speaking North American-accented English," Comey said. And investigating him further "is a big focus of ours," he added, according to ABC News.