FBI agents searched a Phoenix apartment Monday as part of the investigation into the shooting, which occurred Sunday evening in Garland, a city near Dallas.
The FBI had begun monitoring Simpson again recently. Authorities said Monday that they were still working to determine details about the two men and who may have instigated the plan. The FBI does not believe the shooting was directed by an international terrorist group, but is still investigating.
Police say the two gunmen drove up to the Curtis Culwell Center during a controversial art exhibition and, at around 6:50 p.m., opened fire with assault rifles, striking a security guard in the leg.
An officer who normally works on traffic was there as part of a heavy security detail for the event, and this officer shot and killed both gunmen using his duty pistol, said Joe Harn, a spokesman for the Garland police.
“Both those men died there on the street next to their car,” Harn said during a news conference Monday morning.
The gunmen had additional ammunition as well as suitcases in their vehicle, but no bombs or explosives were found, Harn said. They were also wearing some form of protective gear, he said.
While Harn would not say if police believed the shooting was directly connected with the event, he said the two gunmen intended to get inside the center and shoot people.
“We were able to stop those men before they were able to penetrate the area and attempt to shoot anyone else,” Harn said.
The guns recovered by authorities in Garland were being traced Monday by the Bureau of Alcohol, Tobacco, Firearms and Explosives.
City officials said that Bruce Joiner was the Garland Independent School District security officer who was shot at by the two gunmen. Joiner was treated for an ankle wound and released, the city said in a statement.
The inflammatory event was hosted by a New York group that had promised $10,000 for the best cartoon depicting Muhammad. Drawing or otherwise depicting Muhammad is largely forbidden under Islam.
Authorities said there was considerable discussion on social media in the days leading up to the cartoon event, which prompted authorities to deploy heavy security. One tweet sent out on Sunday referred to taking a pledge of allegiance and referred to a #texasattack, but it was unclear whether the account belonged to either of the men involved.
Simpson was born in Illinois and converted to Islam at a young age, court documents show. The government began investigating him in 2006, recording conversations between him and a paid informant.
In May 2009, according a federal court document, Simpson told an FBI informant: “It’s time to go to Somalia, brother.” He added: “It’s time. I’m tellin’ you man. We gonna make it to the battlefield…it’s time to roll.”
Simpson was arrested by the FBI in January 2010 after a lengthy investigation. He was charged with lying to agents in connection with terrorism. Authorities suspected he was trying to fly to Somalia, but Simpson claimed at the time he had intended to travel to South Africa to go to school and study Islam there.
Following a bench trial, a judge dropped the terrorism enhancement, citing insufficient evidence. The judge, Mary H. Murguia, said in March 2011 that the government had failed to prove that Simpson intended to wage violent jihad in Somalia. Murguia reduced the charge to making a false statement to federal officials and sentenced Simpson to three years of probation. Authorities also returned his passport, which they had confiscated after his arrest.
Simpson’s lawyer described him as a very religious man who had converted to Islam.
“He didn’t seem to me to be any threat to anybody,” Kristina Sitton, Simpson’s lawyer, said in a telephone interview Monday. “He seemed to be very kind but entrenched in Islam. He wouldn’t shake my hand.”
She said that after he was sentenced to probation, Simpson called her saying that he had tried to board a domestic flight and was told he could not fly. Sitton said she believed he was on the no-fly list.
Soofi was a pre-med student at the University of Utah from 1998 to 2003, according to a spokeswoman for the school. He left the school in the summer of 2003 without having earned a degree, she said.
Sari Horwitz contributed to this report.
[This post has been updated. First published: 10:49 a.m.]