They wore body armor. They carried assault rifles. And one had declared loyalty to ISIS.

A day after police killed two gunmen who tried to ambush a Garland, Texas, event featuring controversial cartoons of the Muslim Prophet Mohammed, details began to emerge about the shooters.

One suspect, identified as Elton Simpson by a federal law enforcement source, linked himself to ISIS in a tweet posted just before the attack.

He also was no stranger to federal investigators. In 2011, he was convicted of making a false statement involving international and domestic terrorism.

The other suspect, identified as Nadir Soofi by two federal law enforcement officials, was Simpson's roommate in a Phoenix apartment.

He wasn't well-known to federal law enforcement and was not on the FBI's radar, one of the officials said. Investigators were combing through evidence retrieved from the shooters' Arizona home to help piece together a timeline of how their plot came together, the official said.

Authorities are still trying to determine the suspects' motives, U.S. Secretary of Homeland Security Jeh Johnson said Monday. At this point, he said, one thing appears clear: A quick-thinking police officer "likely saved a number of innocent lives."

Simpson and Soofi never made it inside the Curtis Culwell Center in Garland, where in addition to the cartoon contest, a right-wing Dutch politician who's on an al Qaeda hit list was speaking Sunday evening.

A traffic officer working after-hours as security for the event and armed only with a service pistol killed both men, who were wearing body armor and carrying assault rifles, Garland Police Department spokesman Joe Harn told reporters Monday.

"We think their strategy was to get into the event center, and they were not able to get past our perimeter that we had set up," Harn said.

In addition to the officer, who used a .45 caliber Glock, four SWAT team members with high-powered rifles also fired at the suspects, according to a source familiar with the officers involved in the shooting.

"They faced death head-on and, with incredible skill and bravery, were able to save a lot of people," said Zach Horn, an attorney for the officers.

An unarmed security officer working with the patrol officer was shot in the ankle, police said. None of the approximately 200 people attending the event was hurt.

Harn declined to call the incident a terror attack, saying the motive was still under investigation.

"We don't know their intent, other than that they were willing to pull up and shoot police," Harn said.

Links to ISIS?

Investigators haven't revealed what they found in the suspects' apartment, but Simpson's social media footprint reveals one possible motive; he linked himself to ISIS in a tweet posted just before the attack.

"May Allah accept us as mujahideen," the tweet said, adding that Simpson and his fellow attacker had pledged loyalty to "Amirul Mu'mineen" (the leader of the faithful) -- a description that CNN terrorism analyst Paul Cruickshank said likely refers to ISIS leader Abu Bakr al Baghdadi.

After the shooting, an ISIS propagandist that Simpson had earlier asked his readers to follow tweeted, "Allahu Akbar!!!! 2 of our brothers just opened fire" at the Texas event.

"If there is no check on the freedom of your speech, then let your hearts be open to the freedom of our actions," tweeted the propagandist, who was identified by two American groups that monitor jihadi websites as Junaid Hussain, a British ISIS fighter in Syria who goes by the name Abu Hussein al Britani.

In 2011, Simpson was sentenced to three years of probation after his conviction on the terror-related charge, court records show. Prosecutors said he told FBI agents that he had not discussed traveling to Somalia to engage in "violent jihad" when, in fact, he had, according to an indictment reviewed by CNN.

U.S. authorities are investigating whether Sunday's shooting has any link to international terrorism. Simpson's tweet could indicate the attack was inspired by ISIS, but not necessarily orchestrated by the group, sources said.