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.
Similarities to attacks in Denmark, France
The incident bears similarities to attacks this year on events in France and Denmark featuring images of Mohammed, which some Muslims believe is blasphemous.
In January, gunmen attacked the offices of Charlie Hebdo
, a French satirical magazine that has a controversial history of depicting Mohammed, and killed 12 people. The next month, a gunman attacked a free speech forum in Copenhagen
, Denmark, featuring cartoonist Lars Vilks, who infuriated al Qaeda with his depictions of Mohammed.
The Sunday night event in Garland invited cartoonists to send in caricatures of Mohammed. It was organized by the American Freedom Defense Initiative -- considered an anti-Muslim group by the Southern Poverty Law Center
, which tracks hate groups.
The keynote speaker was right-wing Dutch politician Geert Wilders, who's on an al Qaeda hit list.
Organizers said they received more than 350 submissions for the event.
The winning entry won $12,500. The black and white drawing shows a cartoonist's hands sketching a sword-wielding Mohammed, who is shouting, "You can't draw me!"
A speech bubble coming from the hands depicts the cartoonist's response: "That's why I draw you."
"The Islamic jihadis are determined to suppress our freedom of speech violently." Pamela Geller, president of the American Freedom Defense Initiative, told CNN. "They struck in Paris and Copenhagen recently, and now in Texas."
Responding Monday to criticisms of her group as anti-Islamic, she said, "There is a problem in Islam
, as illustrated last night, and anyone that addresses it gets attacked in this same way."
Venue hosted anti-Islamaphobia event
The American Freedom Defense Initiative said it specifically picked the venue for Sunday's event, a school district-owned facility, because it had hosted an event denouncing Islamophobia in January.
SoundVision, the organizers of the January "Stand with the Prophet" event, denounced Sunday's attackers and also criticized Geller's organization for "hate mongering."
"Unfortunately, some insane persons, however, decided to give hate-mongers the attention they desired with their violent act. ... Once again, a bad name for the community," SoundVision said on its website
. "We, the people of faith, must counter the war-terror-hate cycle with peace-love-respect."
Shortly after the Sunday night shooting, a prominent Muslim leader in Dallas tweeted about it.
"The community stayed away from event," Imam Zia Sheikh wrote. "Seems like a lone wolf type of attack. Just what we didn't want."
'A gentle person'
Members of a mosque the suspects attended, the Islamic Community Center of Phoenix, are in shock about what happened, said its president, Usama Shami.
Simpson was a regular worshiper at the mosque until around 2010 or 2011, about the time the FBI arrested him on the false statement charges.
During that time, he offered no signal that he held radical views, Shami said.
"He was a gentle person," Shami said of Simpson. "He always had a good attitude, a good demeanor."
Soofi came to the mosque less frequently, Shami said. He owned a local pizza shop and would show up at the mosque with his young son.
"They didn't show any signs of radicalization or any signs of even thinking about those things in that manner. So when that happens it just shocks you," Shami said. "How good did you know these people, that's the question that people ask themselves."
In the 1990s, Soofi spent a few years in Islamabad, Pakistan, where he lived with his father and stepmother and attended a prestigious private school, a source with knowledge of the family said.
His father was Pakistani, the source said, and his mother was American.
'Freedom of speech is under violent assault'
Wilders, the Dutch politician who was the keynote speaker at the Garland event, is controversial for his anti-Islam views. He was placed on the al Qaeda hit list for his film "Fitna."
The film, which Wilders released online in March 2008 to international outcry, features disturbing images of terrorist acts superimposed over verses from the Quran in an apparent attempt to paint Islam as a threat to Western society.
In 2011, Wilders was cleared of charges of inciting discrimination and hatred with the movie.
"The day we give away humor and freedom of speech is the day that we cease to exist as a free and independent people," he told the attendees at the Garland event Sunday night.
The American Freedom Defense Initiative is also known for its anti-Muslim stance, with the Southern Poverty Law Center describing Geller as "the anti-Muslim movement's most visible and flamboyant figurehead," a description that she disputes.
The conservative blogger first gained national attention with her group, "Stop the Islamization of America,"
and its vocal opposition to an Islamic community center planned near the site of New York's ground zero, where the twin towers of the World Trade Center were destroyed by Islamist hijackers on September 11, 2001.
Geller said Sunday's attack showed how necessary the event was, adding that she plans to hold similar events in the future.
"The freedom of speech is under violent assault here in our nation," she said. "The question now before us: Will we stand and defend it or bow to violence, thuggery and savagery?"