FBI agents collect evidence Tuesday from the area around a car driven by two men killed Sunday evening after they opened fire outside a contest for cartoons depicting the Muslim prophet Muhammad at Garland ISD's Culwell Center. The men were identified Monday as Phoenix roommates Elton Simpson and Nadir Soofi. (Nathan Hunsinger - Staff Photographer)

Garland police spokesman Joe Harn answers questions during a press conference Monday morning. Two men were killed Sunday when they opened fire, injuring a Garland ISD police officer, outside a contest for cartoons depicting the Muslim prophet Muhammad at Garland ISD's Culwell Center. (Ron Baselice - Staff Photographer)

FBI agents collect evidence around the car driven by two men killed Sunday when they opened fire outside a contest for cartoons depicting the Muslim prophet Mohammad at Garland ISD's Culwell Center. (Ron Baselice - Staff Photographer)

A tarp shields the view Monday morning of the car driven by two men killed Sunday in Garland after they opened fire outside a contest for cartoons depicting the Muslim prophet Muhammad. The car was dismantled to ensure that it was not carrying explosives. (Ron Baselice - Staff Photographer)

Garland officers kept attendees inside the Curtis Culwell Center after the shooting. (Nomaan Merchant - The Associated Press)

Police officers stands guard at a parking lot near the Curtis Culwell Center where a provocative contest for cartoon depictions of the Prophet Muhammad was held Sunday, May 3, 2015, in Garland, Texas. The contest was put on lockdown Sunday night and attendees were being evacuated after authorities reported a shooting outside the building. (AP Photo/LM Otero) (LM Otero - AP)

Police outside of an evacuated Walmart on North Garland secure a perimeter after a shooting during the American Freedom Defense Initiative program at the Curtis Culwell Center on Sunday, May 3 2015 in Garland, Texas. (Gregory Castillo/The Dallas Morning News) (Gregory Castillo - Staff Photographer)

Bosch Fawstin (left), the winner of the cartoon contest, was presented with a check by Dutch politician Geert Wilders and Pamela Geller, the organizer of the event in Garland, on Sunday. (Gregory Castillo - Staff Photographer)

The Latest:

Garland police praise a quick-thinking officer for shooting and killing the gunmen, identified as Elton Simpson and Nadir Soofi.

FBI agents have searched the apartment the two men shared in Phoenix.

The president of a Phoenix mosque said Soofi and Simpson used to attend but hadn't lately.

A news conference scheduled for 3:30 p.m. was canceled shortly after 2:30. Police say they have nothing new to report.

A quick-thinking Garland police officer shot and killed two gunmen who opened fire at a controversial art exhibit featuring cartoons depicting the prophet Muhammad, police said Monday.

Police say the two gunmen were armed with assault rifles and wearing body armor when they pulled up to the Curtis Culwell Center shortly before 7 p.m. Sunday. They began shooting almost immediately and wounded an unarmed Garland ISD security guard who was with the officer. The officer returned fire and killed the gunmen.

"Under the fire that he was put under, he did a very good job and probably saved lives," said police spokesman Joe Harn. “We had a SWAT react team in the back that very quickly responded within seconds and helped secure the scene.”

The officer’s name hasn’t been released. The security officer, Bruce Joiner, 58, was treated at a hospital for an ankle wound and released. He is a former Rowlett police officer.

Authorities were investigating whether the shooting was a terrorist attack.

"This is not going to be a real fast investigation,” Harn said Monday.

For hours overnight, a bomb squad investigated whether explosives might be in the car the two suspects drove up in. They found suitcases and additional ammunition, but no bombs.

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Texas Gov. Greg Abbott said he has received a briefing about the incident from the Texas Department of Public Safety, which is working with federal authorities on the investigation. Abbott said he called Garland Mayor Douglas Athas and offered any state resources and help.

One of the gunmen killed Sunday has been identified as an Arizona man who was the subject of a jihadist terrorism investigation. A senior FBI official confirmed one shooter was Elton Simpson, 31, whose Phoenix apartment was being searched by federal agents Monday morning, according to ABC News.

The second shooter was identified as Nadir Soofi, 34, according to The Washington Post. The shooters were roommates, according to reports.

Who instigated the plan remains to be seen, The Post said, but the FBI had no reason to believe an international terror group directed the attack.

Five years ago Simpson was convicted for lying to federal agents about talking about traveling to Somalia in 2009 to engage in “violent jihad.” Simpson talked about “the obligation to fight jihad overseas” in conversations the FBI taped, court records show.

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In 2010, when the FBI questioned him at his home in Avondale, Ariz., Simpson had a plane ticket to travel to South Africa as well as his passport. Authorities believe Simpson was also behind Twitter messages sent shortly before the Sunday attack that included the hashtag "#texasattack."

“May Allah accept us as mujahideen [those engaged in jihad],” the tweet said.

His father, Dunston Simpson, told ABC News on Monday that his son "made a bad choice."

"We are Americans and we believe in America. What my son did reflects very badly on my family," Simpson told ABC News.

Neighbors in the Phoenix apartment complex where Soofi and Simpson lived said they never saw anything out of the ordinary from them. A woman said she had a few interactions with one of the men, who she said smoked marijuana.

Soofi was convicted of a misdemeanor marijuana possession last month, according to Arizona court records.

"I was never threatened by him. He seemed pleasant," said the woman who asked not be identified because of the nature of the investigation.

Another neighbor, Tim Rains, said one of the men helped him from his car after he had trouble breathing a year ago. "He came right up and offered to help," Rains said.

Soofi and Simpson once attended the Islamic Center of Phoenix but stopped coming recently, said Usama Shami, president of the mosque about 15 minutes north of downtown.

Shami said he met Simpson 10 years ago at a time when it was suspected that an FBI informant was a member of the mosque. Federal court documents confirm that the FBI used a confidential informant in 2006 to help build the government's case against Simpson.

Shami said the mosque doesn’t encourage violence and said the men’s possible radicalization would have been corrected had they stayed.

“If you look at people who have been radicalized, they haven’t been radicalized in the mosque,” Shami said Monday at the mosque, which is about 15 minutes north of downtown Phoenix.

Sunday's controversial event included an art show of Muhammad caricatures, which many Muslims consider offensive. The event was hosted by Pam Geller, whose group, the American Freedom Defense Initiative, is considered an anti-Muslim organization.

The group offered a cash award for best drawing and defended the event as their freedom of expression. The organization said it spent $10,000 on extra security.

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The White House on Monday condemned the shooting but stopped short of criticizing the event.

“We have seen extremists try to use expressions that they considered to be offensive as a way to justify violence not only in this country but around the world," press secretary Josh Earnest told reporters aboard Air Force One. "In the mind of the president there is no form of expression that would justify an act of violence.”

Two prominent Muslim imams in the Dallas area on Monday denounced the violence in Garland and called on community members "not to be baited" into anger by events like the "Muhammad Art Exhibit."

“As a Muslim community we need to acknowledge hate groups and not get baited,” said Imam Zia ul-Haque Sheikh, the leader of one of North Texas’ largest mosques. “They are trying to provoke the Muslims into doing something wrong.”

Imam Zia, who leads the Islamic Center of Irving, added, “The public really needs to know that we stayed away from the event and did not give it publicity.”

Police and federal authorities were still sifting through evidence on Monday outside the Culwell Center. For hours after the shooting, authorities kept the estimated 200 people there inside. They were eventually evacuated and taken to a secure location for questioning.

Their cars remained at the center on Monday, as was the vehicle belonging to the suspects. The trunk of their car was opened like a tin can after a police bomb squad used explosives to get inside.

"There were several detonations of things that we were finding," Harn said Monday. "One of the first detonations you heard was to open the trunk of the vehicle."

Jim Harman, who was inside the center, said that police kept attendees calm when the shooting began. He said security guards quickly escorted Geller from the stage.

“We knew something had happened, but we didn’t hear anything and didn’t see anything,” he said.

Before the shootings, Geller dismissed critics who called her American Freedom Defense Initiative an extremist organization.

“As if pursuing good in the extreme makes it a bad thing,” she said, adding that the shooting Sunday showed how “needed our event really was.”

Staff writers Claire Z. Cardona, Julie Fancher, Naomi Martin, Robert Wilonsky, Kevin Krause, Tasha Tsiaperas and Matthew Haag and the Arizona Republic and The Associated Press contributed to this report.

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