Douglas McAuthur McCain, a 31-year old American from San Diego, has been killed in fighting between rival Islamist groups. The State Department confirmed McCain's death and that he died while fighting for Islamic State.

NBC News:

Douglas McAuthur McCain, of San Diego, California, was killed over the weekend fighting for the Islamic State of Iraq and al-Sham (ISIS), according to the Free Syrian Army. Photos of McCain's passport and of his body -- which feature a distinctive neck tattoo -- have been seen by NBC News. According to an activist linked to the Free Syrian Army who also saw the body and travel document, McCain was among three foreign jihadis fighting with ISIS who died during the battle.

Senior administration officials told NBC News they were aware that McCain was killed in Syria and that his family was informed on Monday. The officials added that they believe dozens of Americans have gone to Syria to fight with extremist groups - including, but not limited to, ISIS.

"The threat we are most concerned about to the homeland is that of fighters like this returning to the U.S. and committing acts of terrorism," a senior administration official told NBC News.

National Security Council spokeswoman Caitlin Hayden later confirmed the death, adding, "We continue to use every tool we possess to disrupt and dissuade individuals from traveling abroad for violent jihad and to track and engage those who return."

NBC News has contacted several members of McCain’s family and dozens of friends – including his mother, sister, aunts and cousins. A woman who said she was McCain's aunt confirmed that he had "passed" and referred calls to McCain’s mother.

McCain, 33, called himself "Duale ThaslaveofAllah" on Facebook and his Twitter bio reads: "It's Islam over everything." But how did this would-be rapper go from shooting hoops in a blue-collar area of Minnesota to dying in the civil war thousands of miles from home? This is his story -- one shared in part by only a "small handful" of Americans believed to be fighting in Syria with ISIS.

McCain led an unremarkable life. There's no indication from his past that he would become a violent jihadist. It's unclear where or when he was radicalized as friends say that when he converted to Islam in 2004, he didn't change very much. Religion was rarely mentioned on his social media sites.

Where Great Britain and western European countries have dozens, even hundreds of their citizens fighting for ISIS and other jihadist groups, the US has very few turncoats. Part of the reason is that Europeans tolerate radical mosques much more than we do. Turning disaffected Muslim kids into killers at these mosques is part of an organized recruiting effort by jihadist groups like ISIS.

Authorities were unaware of any individual Americans who had joined the fight in Syria against Assad, which is why they believe only a handful were involved.

But it only takes one, smiling American jihadist with an automatic weapon in a mall or subway to commit an horendous crime.