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.
Joe Harn, a Garland, Texas, police officer, said one officer returned fire with his service pistol, killing both suspects.
At a news conference, Harn added details about the incident, which 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 terror.
"We're certainly looking into that," he said. "We have not knocked that out."
Harn refused to identify the suspects in case, saying the FBI and the ATF were helping local law enforcement with the ongoing investigation.
From its inception, the event was controversial. The main organizer, Pamela Geller, has been threaten by Muslim extremists in the past and the organization paid $10,000 to have extra security on hand.
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 that a threatening tweet 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.
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