May 4, 2015, 12:48 AM UTC / Updated May 4, 2015, 7:48 AM UTC / Source : NBC News

Two gunmen were killed and a security guard wounded Sunday outside a "Draw Muhammad" contest organized by a free speech group at a school arena in Garland, Texas, police said.

The attackers drove up and opened fire on the security guard as the event was finishing up about 7 p.m. (8 p.m. ET), said Joe Harn, a spokesman for the Garland police.

Officers at the heavily-policed event returned fire, killing the men.

The suspects' bodies remained at the scene because investigators were concerned there could be a bomb in their car. A bomb squad robot was checking the vehicle, Harn said.

There were no immediate clues to the identity of the attackers.

The area was evacuated, and the 200 or so participants were taken away from the scene on buses, said Harn.

The Garland Independent School District identified the unarmed security guard as Bruce Joiner, who was treated for a gunshot wound to the ankle and released from a hospital several hours later.

School tests due to take place at the center Monday would no longer be held there, it said.

The FBI was involved in the investigation and the area would remain an active scene until the morning, NBCDFW reported.

The contest was described as a "free speech" event by its sponsor, the American Freedom Defense Initiative, which is listed by the Southern Poverty Law Center as an active anti-Muslim group. It offered a $10,000 top prize and featured Dutch politician Geert Wilders, who is known for his anti-Muslim views, as its speaker.

Dutch politician Geert Wilders speaks Sunday at the Muhammad Art Exhibit and Contest in Garland, Texas. MIKE STONE / Reuters

Garland police are working to secure the scene after 2ppl shot at event showcasing cartoons of Muhammad @NBCDFW pic.twitter.com/2gqTx0ojsw — Jocelyn Lockwood (@JocelynNBC5) May 4, 2015