The City of Garland said in a statement posted on its Facebook page Sunday night that two men drove up to the Curtis Culwell Center and began shooting at a security officer.
Police officers engaged the gunmen, who were both shot and killed, the statement said.
The statement did not say whether the shooting was related to the event. The security officer's injuries were not life-threatening, the city said.
The gunmen's vehicle may contain an "incendiary device," according to the statement. A bomb squad was on the scene, and nearby businesses were evacuated.
Police blocked off a large area around the center Sunday night. There was a heavy police presence, and at least three helicopters circled overhead.
The New York-based American Freedom Defense Initiative had been hosting a contest at the center that would award $10,000 for the best cartoon depicting the Prophet Muhammad (PBUH).
Such drawings are deemed insulting to many followers of Islam and have sparked protests around the world. According to mainstream Islamic tradition, any physical depiction of the Prophet Muhammad (PBUH) is considered blasphemous.
Earlier Sunday, about 75 attendees at the contest were escorted by authorities to another room in the conference center. They were then taken to a separate location, where an Associated Press reporter was told they could not leave until FBI agents arrived to question them.
Neither the FBI office in Washington, D.C., nor the Dallas office had any immediate comment.