The FBI had spent nearly a decade investigating one of the men who attempted to attack an event featuring cartoons of the Muslim prophet Muhammad, it was revealed Monday as investigators attempted to determine whether the attack was directed by any overseas terror groups.

That suspect, identified by authorities as Elton Simpson, was killed by police officers in the Dallas suburb of Garland, Texas Sunday evening after shooting a school district security guard in the leg outside a center where a cartoon contest was being held. Simpson's fellow attacker and roommate, identified as Nadir Soofi, was also killed. 

Authorities said Simpson and Soofi carried rifles and were wearing body armor. Garland police spokesman Joe Harn said Monday that a single Garland police officer subdued the two gunmen but that after his initial shots, SWAT officers nearby also fired at the two men. Harn said police don't know who fired the lethal shots.

A convert to Islam, Simpson first attracted the FBI's attention in 2006 because of his ties to Hassan Abu Jihaad, a former U.S. Navy sailor who had been arrested in Phoenix and was ultimately convicted of terrorism-related charges, according to court records. Jihaad was accused of leaking details about his ship's movements to operators of a website in London that openly espoused violent jihad against the U.S.

In the fall of that year, the FBI asked one of its informants, Dabla Deng, a Sudanese immigrant, to befriend Simpson and ask for advice about Islam. Deng had been working as an FBI informant since 2005 and was instructed to tell Simpson he was a recent convert to the religion.

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Over the next few years, Deng would tape his conversations with Simpson with a hidden recording device accumulating more than 1,500 hours of conversations, according to court records.

Simpson was arrested in 2010, one day before authorities say he planned to leave for what he said were religious studies at a madrassa in South Africa. But despite the hours of recordings, the government prosecuted him on only one minor charge -- lying to a federal agent. Years spent investigating Simpson for terrorism ties resulted in three years of probation and $600 in fines and court fees.

"I have to say that I felt like these charges were completely trumped up, that they were just trying to cover up what had been a very long and expensive investigation and they just couldn't leave without some sort of charges," Simpson's attorney, Kristina Sitton, told the Associated Press.

Sitton described Simpson as so devout that he would not even shake her hand and would sometimes interrupt their legal meetings so he could pray. She said she had no indication that he was capable of violence and assumed he just "snapped."

In recent years, Simpson, described as quiet and devout, had been on the radar of law enforcement because of his social media presence, but authorities did not have an indication that he was plotting an attack, one federal official familiar with the investigation told the Associated Press.

In a statement released late Monday by Phoenix law firm Osborn Maledon, Simpson's family said it is "struggling to understand" how the incident happened.

"We are sure many people in this country are curious to know if we had any idea of Elton's plans," the statement says. "To that we say, without question, we did not."

The statement, which does not identify the relatives, also says the family is "heartbroken and in a state of deep shock" and sends prayers to everyone affected by this "act of senseless violence," especially the security guard who was injured.

The recordings played at Simpson's trial featured him talking about fighting against non-Muslims, to whom he referred as "kuffar."

"Allah loves someone who is out there fighting [non-Muslims] and making difficult sacrifices such as living in caves, sleeping on rocks rather than sleeping in comfortable beds and with his wife, children and nice cars," Simpson told the informant in a recording played at his trial. "If you get shot, or you get killed, it’s [heaven] straight away…That’s what we here for…so why not take that route?"

The Dallas Morning News reported that one recording featured Simpson saying his planned studies in South Africa were "just a front" and said he was ready to "bounce" if he had to.

In an apparent reference to the U.S. wars in Afghanistan and Iraq, Simpson said at one point. "They trying to bring democracy over there man, they’re trying to make them live by man-made laws, not by Allah’s laws. That’s why they get fought. You try to make us become slaves to man? No we slave to Allah, we going to fight you to the death."

"I'm telling you, man, we can make it to the battlefield," Simpson is recorded saying on May 29, 2009. "It's time to roll."

Simpson, a longtime resident of the Phoenix area, had worshipped at the Islamic Community Center of Phoenix for about a decade, but he quit showing up over the past two or three months, the president of the mosque told The Associated Press.

Simpson was quiet, never angry and a regular on the basketball court playing with young members of the mosque, said Usama Shami. He asked questions about prayer and marriage, Shami said. And he was rattled by the FBI investigation into him years earlier. Shami said most people at the mosque knew Deng was an informant because he showed such little interest in learning about Islam.

"I've never seen him angry," Shami said of Simpson. "That's the honest truth. He was always having a grin."

Less was known about Soofi, who appeared to have never been prosecuted in federal court, according to a search of court records.

Sharon Soofi, his 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 said the last time she had communicated with her son was last month, sending a text to wish her grandson a happy birthday.

"He put his son above everything, I thought," she told the newspaper. "The hard thing is to comprehend is why he would do this and leave an eight-year-old son behind."

The Associated Press contributed to this report. 

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