US: American Citizen 'Involved' in Suicide Bombing in Syria

The U.S. State Department today confirmed that an American citizen was "involved" in a suicide bombing attack in Syria, the first time an American is known to have participated in such an attack in the Middle Eastern nation's bloody civil war.

Claims that an American had performed a "martyrdom" operation first surfaced online earlier this week from the powerful al Qaeda-linked rebel group Jabhat al-Nusra. The American, whose purported nom de guerre was Abu Huraya al-Amriki, was one of four suicide bombers used in a coordinated attack by the rebel group against government forces, the group said Tuesday on Twitter.

The tweet included a picture of the alleged American bomber along with the truck used in the attack. Another image released online, which appeared to possibly have been digitally manipulated, supposedly shows the American, a bearded, light-skinned man kneeling on the ground wearing what appears to be a suicide vest. A video of the bombing also circulated in jihadi circles online, but the suicide bomber is not identifiable in that video.

U.S. government sources told ABC News Thursday that American intelligence believed al-Nusra's claim to be accurate, as first reported by NBC News, and that the American was from Florida. The State Department's comment today marks the first public confirmation.

WATCH: Rising Threat, American Suicide Bombers

The unidentified American joins a tiny group of U.S. citizens that have conducted suicide bombings. Three Americans have participated in suicide attacks on behalf of the al Qaeda affiliate al-Shabab in Somalia since 2009, the first of which sent a "shockwave" through the American counter-terrorism community, senior counter-terrorism officials told ABC News.

Top American officials have said in recent weeks that they are concerned about Americans who travel to Syria to fight in the civil war there - fearing they could return to the states to launch attacks against the homeland. As ABC News reported in January, the FBI is already watching dozens of fighters who have returned from the Middle Eastern battlefield.

READ: From Syria to Stateside, a New Al Qaeda Threat

Last week the Justice Department appointed a special prosecutor to go after these former fighters in an effort to stem the flow of foreign fighters to Syria.

FBI Director James Comey told ABC News earlier this month that his organization aimed to make sure the "coming Syria diaspora" does not turn into a "future 9/11."