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By Tracy Connor

The gunman who killed four Marines in attacks on military centers in Chattanooga had a short-lived career as an amateur mixed-martial arts fighter and is remembered by his trainers as a regular teenager who was a bit "passive" for the combat sport.

Jesse Grun and Jeremiah Gurley said Mohammad Youssuf Abdulazeez joined their former gym, the Chattanooga Fight Factory, right after he graduated from Red Bank High School.

"Seemed like a normal kid back then," Grun told NBC News after Thursday's rampage. "He was a nice kid. Wasn't really shy, but not loud either."

Other acquaintances, including neighbors and former classmates, are also struggling to come to terms with the seemingly well-adjusted Abdulazeez they knew with the image of him killing U.S. service members. Authorities, meanwhile, are searching his history of travel and online activity for clues. So far they've found nothing obvious — no suicide video, no manifesto, no note.

Abdulazeez was on the wrestling team at Red Bank High School. A fellow graduate, Hussnain Javid, told The Associated Press that Abdulazeez was "very outgoing," and that "everyone knew of him."

Afterwards, Abdulazeez wanted to branch out into MMA. His devoutly religious family, however, did not want him to participate in the full-contact sport, which involves grappling, boxing and martial-arts techniques.

"I remember him not telling his parents about the fight and they ended up finding out and showing up," Grun said.

The teen won that July 2009 bout, which was captured on video. But it was his last competition because his parents "pulled him out of the gym after that," Gurley said.

Gurley said he was shocked when he found out Abdulazeez was accused of killing four Marines and shooting a cop in what authorities are investigating as an act of domestic terrorism.

"He seemed like a really good kid. He was kind of passive, actually," he said.

Grun said he had not seen Abdulazeez since he left the gym, which later closed. He said he wondered what had happened in the intervening six years to mold him into a killer.

"I believe he was still an open-minded, young happy teenager, so something must have changed as he grew into an adult," Grun added. "I'm sad that the gym closed and he might not have had good, positive people around him."

Abdulazeez was born in 1990 in Kuwait, and he was a naturalized U.S. citizen. He lived with his parents, and his father is a Chattanooga city employee. The Kuwaiti Interior Ministry said Friday that Abdulazeez was of Jordanian origin and only born in Kuwait.

An acquaintance of the suspect, Matt Horton, told NBC affiliate KGW that he last saw him at a Halloween party in 2012 and didn't see any red flags.

"{He was an] introverted guy, but he was really friendly if you sat down and talked to him," Horton said.

"I don't want to see him painted just as a career terrorist, that this is what he was brought up to do."

Kagan Wagner, who attended middle and high school with Abdulazeez, told NBC affiliate KING5 she had a similar impression of him.

"It's an unspeakable thing he did and there's no way it can be explained in any way," she said. "It's awful. But I want people to know he wasn't always like this. He was a completely different person growing up. He was the boy next door."

More recently, Abdulazeez attended University of Tennessee Chattanooga, where Chancellor Steven Angle said he had an unremarkable record before graduating in 2012 with an electrical engineering degree.

"There’s nothing that we have in terms of information about his activities that would have raised a red flag,” Angle said. “But we’re fully cooperating with law enforcement in their investigation to make sure we can answer all and any questions that they raise.”

Resumes posted online show that Abdulazeez had a number of internships, including one with the Tennessee Valley Authority. His hunt for work had taken him to Ohio and Virginia, according to the resumes.

James Novogrod, Pete Williams and Associated Press contributed.