U.S. officials believe they already have the identity of the masked ISIS fighter in the James Foley beheading video released three weeks ago.

The two officials, speaking on condition of anonymity, told CNN that they are not yet 100 percent sure, although authorities are positive of the man's identity. Neither was willing to reveal the identity of the ISIS fighter, citing the ongoing investigation.

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The unnamed U.S. official told CNN that authorities are exhausting all means to identify the person, referring to the sophisticated technologies used to identify the man's voice based on the released Foley beheading video.  

British authorities have already briefed FBI agents who are in the UK now to investigate and arrest the Islamic State fighter's associates, the Daily Mirror reported Monday. According to Fox News, the man has been known by British authorities as a collector for Arab-based NGOs before he travelled to Syria to join the conflict.

On Tuesday, the Daily Mirror reported that the jihadist, dubbed as "Jihadi John" because of his association with three other UK-born ISIS fighters hostages call "The Beatles", had been under the British M15's radar for over a year now.

"John" is the ringleader of the three British jihadists identified to be the main guards of foreign nationals in the ISIS stronghold in Raqqa.

Former Turkish hostages confirmed that the masked militant served as the main negotiator for the Islamic State during talks earlier this year. Another former hostage held in Raqqa for a year told the Guardian that the executioner is educated, intelligent and a devout follower of radical Islam.

Afzal Ashraf, a fellow at the Royal United Services, said the man appeared to be among the 500 British jihadists brutalized by the Islamic State group. He was reportedly considered low-risk at the time he fled UK to fight in Iraq and Syria.

Like a number of other jihadists, Jihadi John seemed to have radicalized overnight before he joined the ranks of the Islamic State militants, the Daily Mirror reported an unnamed source as saying.

Authorities are still trying to confirm if the masked executioner in the recently released beheading video of American-Israeli journalist Steven Sotloff is the same person who appeared in the Foley video, reported CNN.

According to the source, the investigation is still "too premature," but American and British authorities are trying to locate the masked man who is believed to be in Syria.

Foley, an American journalist who worked for the online news outlet GlobalPost, disappeared on November 22, 2012. ISIS posted the video of his gruesome beheading on Youtube on August 19.

Steven Sotloff was writing freelance for Time and Foreign Policy magazines before he was captured in Syria a year ago. The video of his bloody death circulated online on September 2. 

Tagsfoley execution, sotloff beheading, Jihadi John, ISIS, ISIL, Islamic State group, Islamic state of iraq and syria, Islamic State of Iraq and the Levant, UK-born ISIS fighters, British jihadists, james foley, Steven Sotloff