The battle in itself seemed tragically normal. Two Syrian opposition groups fought and there were heavy casualties on both sides. Then victorious rebels rifled through the pockets of the dead. One contained about $800 in cash -- and an American passport.

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.

'He Was a Goofball in High School'

Douglas McAuthur McCain was born in Illinois on Jan. 29, 1981. His family later moved to Minnesota's Twin Cities area where he attended Robbinsdale Cooper High School in New Hope as part of the class of 1999.

Classmates at the school – which was described to NBC News as around 75 percent white and 10 percent African American - recalled an “always smiling” joker who liked to laugh and play basketball. McCain wasn’t on the high school team and didn't come across as religious, according to one basketball buddy.

“He was a goofball in high school,” that classmate told NBC News. “Doug was a fun guy to be around. Played basketball, joked a lot, had a small sense of humor. Got along with most … Wasn't the best athlete, but liked to play.”

Another classmate recalled going to teen nights on Fridays at the local YMCA to play basketball and dance. McCain - who had a younger brother and sister - "was a good guy who loved his family and friends," the classmate said. "He would be the light in anyone's darkness. He always made you smile with his goofy self."

One close friend of nearly 20 years told NBC News he had messaged with McCain via Facebook just last week.

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"It's funny because I said where are you and I never got a reply," the friend said, at times choking back tears. "He didn’t actually respond. Now that I know what I know...Whew. It's just all so weird.”

After high school, McCain stuck around the Twin Cities for at least a while. Public records searches show several run-ins with the law. One mugshot of a Douglas McAuthur McCain details an arrest in 2000 at the age of 19 in New Hope on charges of disorderly conduct. Another arrest record – also from New Hope - shows the same man was arrested again in 2006 and booked on charges of obstruction. The mugshot from that arrest also clearly appears to be McCain – and has the same neck tattoo that is seen in Facebook photos of McCain on his "Duale ThaslaveofAllah" account - and the body found on the Syrian battlefield. NBC News confirmed on Tuesday that he was convicted of both charges.

Friends said he converted to Islam around 2004 - or "reverted," according to McCain's own Twitter feed.

Booking photos from Anoka County in 2003, left, and Hennepin County Sheriff in 2008, right, show Douglas McAuthur McCain. Courtesy KARE

'He Respected My Christianity'

McCain's devotion to his faith was evident to many who met him on his travels, which included at least one trip to Sweden to rap - making friends and basketball buddies along the way. Several Swedes told NBC News that they met him on a visit there – between three and four years ago – when he performed in the town of Vasteras, near Stockholm and attended an underground rap show.

Kevin Törnström Kohlin remembered seeing McCain’s basketball skills on display at a game in Vasteras – and bonding over a shared love of Rick Ross and Ace Hood.

“He's a good dribbler, passer and great at taking care of the phase of the game when he got the ball,” Kohlin told NBC News. “He was like a really nice guy. He smiles a lot and brings a lot of good energy.”

“We would talk just a little bit about religion,” Kohlin added. “He respected my Christianity.”

At some point, McCain moved to California. His Facebook profile states that he studied at San Diego City College; it was not immediately clear if he had graduated.

Until early last year, a Twitter account linked to McCain included mostly mundane messages to friends about basketball – how the Lakers suck, comments about the Chicago Bulls – with only a few messages about Allah or Islam. The most political it got appeared to be a #FreeGaza tweet on Jan 2, 2013. There was also a message posted around the same time, with an image of a woman in a burqa and the words "there is nothin like a mother love." Then the account went silent for more than a year.

Sometime in between, McCain reportedly worked at a Somali restaurant in San Diego – African Spice – and was known around the Masjid Nur mosque, according to an acquaintance.

“He was a normal guy, who was social, open-minded, like to smile always, and always wanted to be a good Muslim,” the acquaintance from the restaurant said.

McCain’s online life also painted the picture of a devout Muslim who deeply loved his family – along with Pizza Hut and hip-hop. His likes on Facebook ranged from “Quaran and Hadith” to “The Khilafah in Universe,” “A Way to Paradise” to “Craziest Street Fights,” “The American Comedy Co.” to “The Black Flag.”

“Allah keeps me going day and night. Without Allah, I am no one,” read one photo post. Others took a darker turn – posts featured the black flag of ISIS and other militant propaganda photos. In September 2010, he posted an ominous image: “They are coming back soldiers of Allah.”

A MySpace profile linked to Duale – and with the same images as elsewhere on social media – contains similar messages, with pictures of the sometime rapper striking poses. In one photo, he clutches a Quran. “The quran is all I need in this life of sin,” reads another caption.

'I'm With the Brothers Now'

When McCain’s Twitter account sprung back into activity on May 13, the messages again spoke of Islam.

“I reverted to Islam 10 years ago and I must say In sha Allah I will never look back the best thing that ever happen to me,” he posted. McCain - or Duale, as he had come to be known - then appeared to strike up online friendships with several self-proclaimed jihadis.

On April 3, McCain retweeted the full English translation of the speech of Abu Muhammad al-Adnani – the spokesman for ISIS.

Soon after, it appears that McCain’s travels took him to Turkey, a common jihadi route into Syria. Three people told NBC News that they met McCain – who they referred to as Duale – three months ago in in the Istanbul neighborhood of Sultanahmet. Two said they met him at the Burger King, where they ate and talked about the NBA.

U.S. officials have said “a small handful” of Americans are believed to be fighting with ISIS. Earlier this month, a new ISIS propaganda video claimed to feature an American citizen. And in July, chilling video emerged purporting to show the first American to carry out a suicide attack in Syria burning his U.S. passport and issuing threats against the West. The video of Moner Mohammad Abusalha, who grew up in Florida, underscored concerns about the flow of foreign fighters to Syria.

Those have been amplified by an ISIS video narrated by an apparent Westerner and showing the beheading of American journalist James Foley - and the story of McCain.

On June 9, Duale wrote to an alleged ISIS fighter on Twitter: “I will be joining you guys soon.” He also asked if another self-proclaimed ISIS fighter had made it to “r town” – an apparent reference to Raqqa, the militants' Syrian stronghold.

Then came another post: “I’m with the brothers now.” Later, he retweeted: “It takes a warrior to understand a warrior. Pray for ISIS.”

His final Twitter post was last Tuesday.

Ben Plesser, Peter Alexander, David Wyllie and Tom Winter of NBC News contributed to this report.