New details have surfaced about the two alleged gunman who opened fire Sunday night outside of a “draw Muhammad” cartoon contest in Garland, Texas. A security guard on duty was wounded in the shooting. The two gunmen, who police say were armed with assault rifles, were shot and killed by a traffic cop on the scene, according to police. Here’s what we know about what happened – and who was involved.

ISIS claims responsibility

The terror group known as the Islamic State of Iraq and Syria has claimed responsibility for the shooting in Garland, Texas, a suburb of Dallas. The event offered a $10,000 prize for cartoons of the Prophet Muhammad, which are considered blasphemous by many in the Muslim community. ISIS said Tuesday in a radio address that its “soldiers” had carried out the attack and suggested more violence could follow, according to NBC News. But the group did not offer any evidence to support its involvement, and it’s possible ISIS is claiming responsibility for propaganda purposes.

Related: Chilling new details about shooting suspect

Who were the gunmen?

Officials have identified the gunmen involved in the shooting as Elton Simpson and Nadir Soofi, who were roommates in an apartment in north Phoenix. It was not immediately clear whether the suspects were linked to international terror groups, but federal agents are investigating.

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, according to NBC News. Feds first started looking into Simpson in 2006. As part of his conviction, Simpson received three years of probation and paid $600 in fines and fees.

Soofi was born to a mother from Texas and a father from Pakistan, and Soofi attended the International School in Islamabad in the 1990s, NBC News reported. Apparently a popular and well-liked student, Soofi sang in a school choir and performed in a production of “Bye Bye Birdie,” playing the role of Conrad Birdie. Soofi’s parents eventually separated and he returned to the U.S., a former classmate told NBC News.

Who organized the event?

The inaugural Muhammad Art Exhibit and Contest offered a $10,000 prize for cartoons of the Islamic prophet — depictions that are considered blasphemous by many Muslims around the world.

The event was sponsored by the American Freedom Defense Initiative (AFDI) and attended by its president and co-founder, Pamela Geller — who is also president of Stop Islamization of America (SIOA). Both are listed as hate groups by the Southern Poverty Law Center (SPLC).

Geller, a prominent campaigner against plans for a mosque near Ground Zero, is author of “The Post-American Presidency: The Obama Administration’s War on America.” In 2013 she was reportedly barred from entering Britain, where she was due to speak at a rally organized by the far-right group English Defense League.

“This is a war. This is war on free speech,” Geller wrote on her website Sunday. “What are we going to do? Are we going to surrender to these monsters? Two men with rifles and backpacks attacked police outside our event. A cop was shot; his injuries are not life-threatening, thank Gd [sic]. Please keep him in your prayers. The bomb squad has been called to the event site to investigate a backpack left at the event site. The war is here.”

Geller penned several others posts about the shooting, including claiming the British newspaper The Daily Mail blacked out cartoons of Muhammad from the event. “The cowardice of the enemedia has reached monstrous proportions. They will stop at nothing to appease bloodthirsty jihad terrorists. They are not journalists. They are water-carriers for the forces of oppression, hatred, and forcible censorship,” she wrote.

Related: ISIS claims responsibility for attacks

The SPLC describes Geller as the “anti-Muslim movement’s most visible and flamboyant figurehead,” adding, “she makes no pretense of being learned in Islamic studies.”

Geller’s group, the American Freedom Defense Initiative, also gained infamy in 2012 for an advertisement in the New York City subway system that criticized Islam. “In any war between the civilized man and the savage, support the civilized man. Support Israel. Defeat Jihad,” the ad said.

Among the speakers present was outspoken anti-Muslim Dutch politician Geert Wilders, who later tweeted a picture of himself in front of what he called a “SWAT team” of armed police officers.

What happened?

Officials said about 200 people attended the heavily policed event at the Curtis Culwell Center in the Dallas suburb of Garland. As the event was drawing to a close, two gunmen armed with assault rifles drove up to the venue and began shooting, wounding school district security guard Bruce Joiner in his lower leg. Garland police officers returned fire, killing the men.

The name of the officer who took down the gunmen was not immediately released. “He did what he was trained to do,” Garland police spokesman Joe Harn told reporters Monday. “Under the fire that he was put under, he did a very good job. And probably saved lives.”

A video live stream from inside the event showed participants being told by armed police to move into a secure room away from the front of the building. Shoppers at a nearby Walmart were ordered away from the scene by police.

Harn said that a SWAT team responded to the scene within seconds. The gunmen were killed next to their car, Harn said, noting that there were multiple shots fired at the scene. “There’s a lot of casings out there,” he said.

When asked whether the shooting was considered a terror attack, Harn said “we’re certainly looking into that. We have not knocked that out.”

Harn added that he did not expect any other immediate arrests in the case.

Why was it held at a school arena?

The Curtis Culwell Center was chosen because it had previously hosted an Islamic conference — Stand With the Prophet in Honor and Respect — in January. That event was held one week after gunmen in France attacked and killed 12 people at the Paris offices of Charlie Hebdo, a satirical magazine that had published cartoons depicting Muhammad. The AFDI picketed the January conference and moved to hold its own event.

Some had questioned whether the Garland Independent School District, which runs the venue, should allow the event to go ahead.

However, school board President Rick Lambert told the Dallas Morning News that the convention center was a public facility and the district could not discriminate based on viewpoints. The AFDI paid approximately $10,000 for additional security at Sunday’s cartoon contest, the newspaper reported.

NBC News contributed reporting.