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.
Texas Gov. Gregg Abbott issued a statement late Sunday calling the shooting "senseless." He said Texas authorities were "actively investigating to determine the cause and scope" of the attack.
"This is a war. This is war on free speech. What are we going to do? Are we going to surrender to these monsters?" Pamela Geller, the executive director of the American Freedom Defense Initiative, wrote on her website. "The war is here."
"Right when we were beginning to drive away, we heard gunshots," Cynthia Belisle, who attended the event with a friend, told NBC News. "We thought they were fireworks, but they were not."
Belisle said that dozens of cops sprang into action and that she was told to stay put.
She said she heard a second round of shooting and saw police rushing toward the far end of the parking lot with arms drawn.
"We were scared," she said.
The arena hosted a "Stand With the Prophet" event in January. It held that event as well as the "Draw Muhammad" event because it has a non-discriminatory policy, according to The Dallas Morning News.
In January, gunmen in France attacked and killed 12 people at the Paris offices of magazine Charlie Hebdo, after it published satirical cartoons depicting Islam's Prophet Muhammad, which has come to be considered blasphemous by many Muslims around the world.