(This post was last updated at 7:51 p.m. ET.)
Two suspects jumped out of a dark-colored vehicle and began firing assault rifles on Sunday at the site of a contest where participants drew the Prophet Muhammad.
A security officer was wounded; his injuries are not life threatening, officials said.
Joe Harn, a Garland, Texas, police officer, said one officer returned fire with his service pistol, killing both suspects.
At a news conference Monday, Harn added details about the incident that happened at the parking lot of the Curtis Culwell Center in Garland. The American Freedom Defense Initiative, which the Southern Poverty Law Center considers an active anti-Muslim group, had gathered for what it called a free-speech event.
Harn said the event was just about over when the two men pulled into the parking lot. He said the men wore some kind of body armor and that police are still investigating a motive.
Harn was asked if police considered the attack an act of terrorism.
"We're certainly looking into that," he said. "We have not knocked that out."
Harn refused to identify the suspects in the case, saying the FBI and the ATF were helping local law enforcement with the investigation.
Federal law enforcement sources, however, tell NPR's Dina Temple-Raston that one of the suspects is Elton Simpson, an Arizona man who converted to Islam.
According to court records, Simpson was convicted of lying to federal authorities in 2011. The FBI field office in Phoenix investigated whether Simpson might travel to Somalia to fight Jihad.
Over the course of years, they recorded his conversations and heard Simpson talk frequently about jihad and about leaving the United States to go fight for his "brothers in like Palestine."
At another point, investigators heard Simpson talking to an associate about leaving to Somalia, saying nonbelievers are "fighting against us ... because they don't want us to establish sharia."
Several media reports identified the second gunman as Simpson's roommate, Nadir Soofi. NPR has not independently confirmed the name.
From its inception, the event in Texas was controversial. The main organizer, Pamela Geller, has been threatened by Muslim extremists in the past, and the organization paid $10,000 for extra security.
Harn said that meant that local police, the FBI, the ATF and a SWAT team were on hand when the suspects opened fire.
He said police are aware of a tweet that was sent right before the attack. The account had previously sent this message: "When will they ever learn? They are planning on selecting the best picture drawn of Rasulullah (saws) in Texas."
Harn said officials have not connected the tweet to the incident.
In a statement, Jeh Johnson, secretary of the Department of Homeland Security, said: "While all the facts are not in yet, last night's attack serves as a reminder that free and protected speech, no matter how offensive to some, never justifies violence of any sort."
He added: "[I]n reaction to last night's attack, we urge that members of the public not misdirect anger and suspicion at those simply because of their religious faith.
"The strengths and heritage of this Nation include our racial, religious and ethnic pluralism, and our acceptance and inclusiveness of others different from ourselves."