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Garland shooting suspect Simpson was on FBI terror list

One was a "peaceful" Muslim convert who lied to FBI agents during a terror probe, while the other was described as a swoon-inducing school "heartthrob."

One was a "peaceful" Muslim convert who lied to FBI agents during a terror probe, while the other was described as a swoon-inducing school "heartthrob" and devoted dad "raised in a normal American fashion."

The roommates from Phoenix — Elton Simpson and Nadir Soofi — allegedly opened fire with assault rifles outside an event featuring caricatures of the Prophet Muhammad in the Dallas suburb of Garland on Sunday night before SWAT officers shot them dead.

ISIS claimed responsibility Tuesday for the attack — calling the duo its "soldiers" — but did not offer any evidence to support its claim.

As federal investigators look into whether the two were indeed linked to international terror groups, more details emerged about their background.

Simpson was known to the FBI and was convicted of lying to federal agents five years ago about his plans to allegedly join a terror group in Africa.

"I think that they could have had a reason to at least start investigating him," said Kristina Sitton, the lawyer who represented Simpson during his terror case.

"He was very vocal about being devout. He was very vocal about attempting to convert other people and asking other people what their religion was and why they believed in their religion versus his religion."

Related: ISIS claims responsibility for Texas shooting

The federal probe into Simpson began in 2006. Despite an investigation that involved more than 1,500 hours of recorded conversations — including Simpson's discussions about fighting nonbelievers for Allah and plans to link up with "brothers" in Somalia — he was only convicted of lying to a federal agent.

A judge ruled that prosecutors had failed to prove Simpson planned on joining a terror group when in Somalia, and ordered his passport be returned. Simpson faced three years of probation and $600 in fines and court fees.

"I never saw any indications that he was violent," Sitton said. "In fact, he was always very peaceful with me even to the point where I think I got more riled up about the charges than he did because I thought that they were absolutely outrageous."

She added: "I always saw the peaceful side of him. We would be meeting for hours at a time and he would ask if there was an office where he could go and pray."

A Twitter account apparently associated with Simpson mentioned the shooting 30 minutes before it took place, but officials say they are still trying to determine the amount of planning that went into the attack.

In a statement released through lawyers, his family said they were "heartbroken" and "in a state of deep shock."

"We are sure many people in this country are curious to know if we had any idea of Elton's plans. To that we say, without question, we did not," it said. "Just like everyone in our beautiful country, we are struggling to understand how this could happen.

"As a family we do not condone violence and proudly support the men and women of our law enforcement agencies."

Simpson had worshiped at the Islamic Community Center of Phoenix for about a decade, but stopped showing up over the past two or three months, the president of the mosque told The Associated Press.

He was a regular on the basketball court, playing with young members of the mosque, said Usama Shami. "I've never seen him angry," Shami said of Simpson. "That's the honest truth. He was always having a grin."

Soofi, the son of a Texas mother and Pakistani father, attended the International School in Islamabad where one former student remembered him as "a really nice kid."

"He was a regular guy you went to school with," Mariam Saigol told NBC News. "He was such a nice, sweet, normal kid from a well-to-do family. It's quite shocking to hear this. "

Saigol added: "He was in the school play and the choir. He was a good-looking guy, the school heartthrob. All the girls used to swoon over his American-Pakistani looks. He used to play sports, and was an overall cool kid. We were in the school play, Bye Bye Birdie, together. It was a play about Elvis. He was Conrad Birdie."

A former student who did not want to be identified told NBC News that Soofi returned to the U.S. when his mom Sharon, who was an art teacher at the school, split from his father.

"Imagine going from Islamabad to Utah," the ex-student said. "And dealing with your parents divorce."

Another ex-schoolmate, Kalsoom Lakhani, a social entrepreneur who runs a Washington-based company that helps Pakistani entrepreneurs, said Soofi was "a well-liked popular kid when he lived in Pakistan."

"He was not radically religious in any way," Lakhani told NBC News. "What happened after he moved back to America should be looked at more closely in order for us to better understand what went wrong."

Soofi's mother, who now lives in a small town southwest of Houston, told The Dallas Morning News that she had no idea that he would turn to violence.

She said her son was "raised in a normal American fashion" and "was very politically involved with the Middle East. Just aware of what's going on."

"I don't know if something snapped," she said. She told the newspaper Soofi put his son "above everything."

"The hard thing is to comprehend is why he would do this and leave an 8-year-old son behind," she said.

Soofi's maternal grandmother, Shirley Dromgoole of Garwood, Texas, told NBC station KPRC of Houstonthat her grandson wasn't a terrorist and had to have been under the influence of Simpson.

"He was a good boy every time I saw him. Real responsible. Respectful," Dromgoole said.

Additionally reporting by NBC News' Michael Kosnar and Erin McClam

This story originally appeared on NBC News