NBC News:

A traffic officer working off-duty security fatally shot two men wearing body armor and armed with assault rifles who began shooting outside a Prophet Muhammad cartoon art contest in Garland Sunday, "saving lives," police say.

Officer Joe Harn, with the Garland Police Department, said the officer and an unarmed security guard were sitting in a patrol car blocking an entrance to the Curtis Culwell Center when two men pulled up in a dark-colored sedan Sunday night at about 7 p.m.

As the officer and guard exited the patrol car, two men exited the dark-colored sedan, got behind their vehicle and opened fire on the officers, police said.

The police officer, Harn said, began shooting back with his duty pistol, killing both of the armed men.

"He did what he was trained to do and under the fire he was put under, he did a very good job and probably saved lives," said Harn, who has not yet released the officer's name. "His reaction, and his shooting with a pistol, he did a good job."

The unarmed security guard, 58-year-old Bruce Joiner, was shot in the lower leg as he exited the vehicle, Harn said. He was treated at a hospital and released Sunday night.

There were no other injuries reported in the attack.

During a news conference Monday Harn couldn't confirm if shooting qualified as a terrorist attack. He said the men, whose names are not yet being confirmed by police, were obviously there to shoot people and that the officer saved a number of lives.

Police added that plans put in place months ago allowed the department to stop the men before they were able to shoot anyone else.

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Harn added that off-duty officers work events at the Culwell Center every weekend but that organizers paid the department around $10,000 for additional security that included agents with the ATF and FBI, security officers from the school district, off-duty police officers as well as SWAT officers and the bomb squad.