GARLAND, Texas—Two men were killed Sunday in a Dallas suburb after they opened fire outside a building where an exhibit that featured cartoon drawings of Islam’s Prophet Muhammad was being held, city officials said.

The men drove up to the Curtis Culwell Center in Garland, Texas, where the American Freedom Defense Initiative was hosting an event with an award of $10,000 for the top Muhammad cartoon, and began shooting at an unarmed security officer, according to a Garland city spokeswoman.

Garland police who were helping with event security returned fire, shooting and killing the two gunmen, who weren’t immediately identified. Their bodies remained outside of their car late Sunday as bomb squads moved in to check for explosives.

The incident took only a few seconds, said Joe Harn, the police department’s public information officer. He said both men had been armed but couldn’t provide any information about their identities or weapons.

Garland Independent School District, which owns the event center, identified the security officer as Bruce Joiner, an employee of the school district. He was shot in the ankle and treated and released from a local hospital.