Militants in Syria

A man holds up a knife as he rides the streets of Tabqa with others in celebration after Islamic State militants took over Tabqa air base in Syria on Sunday. (Reuters / August 24, 2014)

A Chicago-born man suspected of fighting alongside Islamic State militants who have seized large areas of Iraq and Syria to the alarm of the Baghdad government and its allies in the West has been killed in Syria, a U.S. official said on Tuesday.

"We were aware of U.S. Citizen Douglas McAuthur McCain's presence in Syria and can confirm his death," U.S. National Security Council spokeswoman Caitlin Hayden said in a statement.

"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," Hayden said.

According to a relative, McCain was born in Chicago and grew up in the Robert Taylor Homes on the South Side before the Chicago Housing Authority building was torn down. He moved with his mother to Minnesota and attended high school there. He later moved to the San Diego area, where he attended community college.

Court records show he had a series of minor run-ins with the law while living in Minnesota, pleading guilty to disorderly conduct and theft, both misdemeanors, as well as several traffic tickets and an infraction for driving without a license.

A national security official who asked not to be named told Reuters that the FBI was investigating McCain's death, and a State Department spokeswoman said officials had been in contact with his family and were providing "all consular assistance."

Family members told the Minneapolis Star Tribune that McCain's mother had been called by a State Department official to report that he had been killed in Syria over the weekend. The New York Times reported that McCain was killed by U.S.-backed anti-Assad rebels.

The Star Tribune said McCain's family had been concerned with his expressions of support of Islamic State militants, and the man's uncle, Ken McCain, told CNN his nephew had converted to Islam from Christianity several years ago and traveled to the region via Turkey to fight as a jihadi.

That could place him among the dozens of U.S. citizens that Attorney General Eric Holder said in July were included in a cohort of roughly 7,000 foreign fighters that U.S. intelligence agencies estimate to be operating in Syria, out of roughly 23,000 violent extremists.

Holder said federal prosecutors had opened fewer than 100 investigations into American citizens who may have traveled to Syria or Iraq to fight.

On a Facebook page identified as belonging to McCain, he referred to himself as Duale ThaslaveofAllah. The Facebook page has since been taken down.

On a Twitter account identified as belonging to McCain, he used the name Duale Khalid and wrote, “It’s Islam over everything.”

The person said he converted to Islam a decade ago: “I will never look back the best thing that ever happen to me,” reads one Twitter message.

The Twitter messages display hostility toward gays, white people and Somali immigrants in San Diego. The messages praise Allah and smoking hookah.

One tweet reads: “It’s funny to me how all these so call Muslim claim that they love Allah but always curse the one who try to implement his laws.”

A retweet from a group called Islamic Freedom reads: “Allah never promised this life was easy, but He did promise that He would be with you every step of the way.”

The Twitter account includes a translation of a speech by Islamic State spokesman Abu Muhammad Al-Adnani.

McCain's death comes after a 22-year-old man from Florida carried out a suicide bombing in Syria's Idlib province in May. Later, a Denver woman was arrested in July on suspicion of trying to fly to Syria to support insurgents, and two men in Texas were taken into custody on similar charges in June.

One of the Texas men was charged with "attempting to provide material support to terrorists," a law that Holder urged other countries to copy as vital to counter terrorism efforts.

The Los Angeles Times and Carlos Sadovi of the Chicago Tribune contributed.