Two men who opened fire Sunday night outside an incendiary art exhibit and cartoon contest of images depicting the Muslim Prophet Muhammad were killed by police, authorities said.

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Police in Garland, Texas, said a Garland Independent School District officer was shot in a parking lot at the Curtis Culwell Center when the men pulled up in a car and opened fire.

A senior FBI official told ABC News that one of the shooting suspects had been a previous subject of a terror investigation and that he had sent tweets before the attack mentioning his plans. Law enforcement is searching his apartment in Phoenix, according to ABC.

A security officer with the Garland Independent School District has been released from the hospital after he was shot in the ankle, according to the Dallas Morning News

Garland police initially suspected the gunmen's vehicle may have contained an explosive and dispatched a bomb squad. Nearby businesses including a Wal-Mart and were evacuated.

The suburban Dallas arena, which was hosting an event sponsored by the American Freedom Defense Initiative (AFDI), was locked down. Attendees were later evacuated.

While in lockdown, audience breaks into song and prayer. National Anthem an God Bless America pic.twitter.com/ufgR8BOOpr — Bob Price (@BobPriceBBTX) May 4, 2015

The event also featured a speech by Geert Wilders, an extreme right islamophobic Dutch politician targeted by al Qaeda.

AFDI president and noted conservative activist Pamela Geller released a statement following the shooting calling the incident a "war on free speech." "What are we going to do?" Geller wrote on her website. "Are we going to surrender to these monsters?"

Rep. Michael McCaul (R-Texas), the chairman of the House Homeland Security Committee, criticized the attack in a statement where he characterized it as an attack on free speech.

"Freedom of expression has again been attacked by fanatics. From the capitals of Europe to the streets of Garland, Texas, we have been confronted by attackers who cannot tolerate our open society," he said.

"But we send a clear message to these extremists: we will not be intimidated by violence, and we will not bow down to terror."

— Ben Kamisar contributed to this report, which was updated on May 4 at 7:32 a.m.