ID=26862329GARLAND — Police in Garland say they believe the two men killed outside a controversial art show Sunday night were there to shoot the approximately 200 people attending the event before a traffic officer intervened.

The suspects were identified as Elton Simpson and Nadir Soofi of Phoenix.

The American Freedom Defense Initiative was hosting the Muhammad Art Exhibit inside the Curtis Culwell Center Sunday. The event featured a contest for cartoon depictions of the Prophet Muhammad and was almost over when the incident occurred.

Garland police spokesman Officer Joe Harn said at a 10 a.m. news conference Monday that security had been ramped up for the controversial event, and a plan had been in place involving the FBI for months.

Organizers of the event paid $10,000 to have extra officers on hand.

According to Harn, the two suspects pulled up to the event in a car, immediately got out with assault rifles and began shooting at a police car that was parked next to a barricade.

Garland Independent School District police Officer Bruce Joiner was hit, but suffered non-life threatening injuries.

Harn said a single police officer initially subdued Simpson and Soofi, but that after his initial shots, SWAT officers also opened fire, killing both men outside their car. He said it was not clear yet who fired the fatal shots.

Attendees of the event were rushed inside the Culwell Center after gunfire was heard. People in a secure room at the center sang patriotic songs.

Harn said investigators spent the night searching the suspects' vehicle for bombs or explosives. None were found, but they detonated the vehicle for safety's sake. Harn said luggage was found inside the vehicle, but it wasn't clear when the suspects arrived in North Texas.

Both suspects' bodies remained at the scene until about 11 a.m. Monday, with dozens of shell casings surrounding them. Both were wearing heavy-duty tactical vests. One had a New York Yankees cap next to him.

The suspects' car was finally towed away around 6 p.m. Monday.

The site outside the Curtis Culwell Center remained an active crime scene, Harn said. The vehicles of the nearly 200 people attending the art show were still at the scene. The attendees were bussed away from the event via Garland ISD school buses on Sunday night.

A 1,000-foot radius around the Walmart nearby was initially shut down and multiple businesses in the area were evacuated. The perimeter has since been lifted.

Advanced placement testing scheduled for Monday at the Culwell Center was canceled, according to Garland ISD, but school was in session Monday morning for a high school nearby.

Plano, Irving, DPS, FBI and ATF are assisting Garland police with their investigation, Harn said.

Buck Revell, a former head of the Dallas FBI office, praised Garland's handling of the situation.

"Garland did a terrific job," said Revell, a terrorism expert. "Obviously, they were concerned, because there rumblings on the Internet about 'It's time for the brothers to come out and show these people they can't say anything about the Prophet or about Islam.'"

Revell, like many others, says this won't be the first nor the last attempt of those trying to violently stifle free speech. He rejected the criticism of those who say the Garland school district shouldn't have allowed the use of its facility for the event.

"If you can't host an effort where there's going to be open speech and debate about controversial issues, then we've already lost our freedom," he said. "They've won without having to put forward their agenda on a legitimate basis, much less on an illegitimate and violent basis."

FBI agents on Monday searched an apartment in Phoenix where they say the suspects lived as roommates.

"The two gunmen who were involved in the shooting in Dallas... were determined to be from Phoenix, located at this property," Assistant Special Agent in Charge John Lannarelli told WFAA's sister station KPNX. "Hence the FBI and Phoenix police were involved in determining where at this location and what evidence may be available that can be used in the Dallas investigation."

One of the suspects has been identified as Nadir Soofi.

The other, identified as Elton Simpson, 30, was previously convicted in a terror investigation but received probation.

Dunston Simpson, Elton's father, told ABC News that his son "made a bad choice."

"We are Americans and we believe in America," he said. "What my son did reflects very badly on my family."

Elton Simpson is also believed to be the man who tweeted several ominous messages before the incident Sunday, using the hashtag #texasattack.

Pamela Geller, president of the American Freedom Defense Initiative and organizer of the art show, referred to it as a "free speech event" on Twitter. One tweet from her after the apparent attack read "this is a war."

Governor Greg Abbott released the following statement on Monday:

"This morning I called Mayor Athas to offer any state resources and assistance that would aid his city following this heinous attack, and praised the City of Garland's swift and effective action to stop these gunmen and protect innocent lives. I also received a briefing from the Department of Public Safety on the latest developments regarding the actions and motivations of the shooters. I instructed Director McCraw to work with federal authorities to fully investigate the assailants' ties to organized terrorist activity, and received assurances that DPS will continue to communicate with my office and the public as additional information can be released."

Dr. Nasim Rehmatullah, National Vice President of Ahmadiyya Muslim Community USA, also released a statement:

"Violence is never an acceptable response to hate speech, no matter how inflammatory and uncivilized that speech is. While we do not yet know what motivated these shooters, we urge calm and defer to local, state, and federal authorities to peaceably and justly resolve this."

ABC News says ISIS followers were calling for attacks leading up to the Garland event.

"The brothers from the Charlie Hebdo attack did their part. It's time for brothers in the US to do their part," one supporter wrote.

"Brothers in Garland Texas Please go to there with your weapons, bombs or with your knives. Threaten your enemies & the enemies of Allaah," another wrote last week.

News 8's Tanya Eiserer contributed to this story.