GARLAND, Texas -- Two gunmen were killed Sunday after opening fire on a security officer outside a provocative contest for cartoon depictions of Prophet Muhammad in Texas, and a bomb squad was called in to search their vehicle as a precaution, authorities said.

The men drove up to the Curtis Culwell Center in the Dallas suburb of Garland as the contest was scheduled to end and began shooting at a security officer, the city of Garland stated. Garland police officers returned fire, killing the men.

"Because of the situation of what was going on today and the history of what we've been told has happened at other events like this, we are considering their car (is) possibly containing a bomb," officer Joe Harn, a spokesman for the Garland Police Department, said at a news conference.

Police are not aware of any ongoing threat and had not received any credible threats before the event, Harn said.

Harn said it was not clear whether the shooting was connected to the event inside, a contest hosted by the New York-based American Freedom Defense Initiative that would award $10,000 for the best cartoon depicting the Prophet Muhammad.

Such drawings are deemed insulting to many followers of Islam and have sparked violence around the world. According to mainstream Islamic tradition, any physical depiction of the Prophet Muhammad -- even a respectful one -- is considered blasphemous.

The Curtis Culwell Center, a school district-owned public events space, was evacuated earlier Sunday, as were some surrounding businesses. Police blocked off a large area around the center Sunday night. There was a heavy police presence, and police helicopters circled overhead as bomb squads worked on the car.

Authorities haven't removed the bodies of the gunmen, who have not yet been identified, Harn said. The bodies are too close to the car to be removed, but that will happen when the scene is clear, he said.

The security officer who was shot worked for the Garland Independent School District, Harn said. He was treated at a local hospital for injuries and was released Sunday night.

After the shooting, about 75 attendees at the contest were escorted by authorities to another room in the conference center. They then were 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 comment.

Johnny Roby of Oklahoma City was attending the conference. He said he was outside the building when he heard about 20 shots that appeared to be coming from the direction of a passing car.

Roby said he then heard two shots. He said he heard officers yell they had the car before he was sent inside the building.

Pamela Geller, president of the AFDI, said before Sunday's event she planned the contest to make a stand for free speech in response to outcries and violence over drawings of Muhammad. Though it remained unclear several hours after the shooting whether it was related to event, she said Sunday night the shooting showed how "needed our event really was."

In January, 12 people were killed by gunmen in an attack against the Paris office of the satirical newspaper Charlie Hebdo, which had lampooned Islam and other religions and used depictions of Muhammad. Another deadly shooting occurred the next month at a free-speech event in Copenhagen featuring an artist who had caricatured the prophet.

Geller's group is known for mounting a campaign against the building of an Islamic center blocks from the World Trade Center site and for buying advertising space in cities across the U.S. criticizing Islam.

When a Chicago-based nonprofit held a January fundraiser in Garland designed to help Muslims combat negative depictions of their faith, Geller led about 1,000 picketers at the event. One chanted: "Go back to your own countries! We don't want you here!" Others held signs with messages such as, "Insult those who behead others," an apparent reference to recent beheadings by the Islamic State militant group.