DALLAS (AP) — An Army veteran wearing a mask and carrying more than 150 rounds of ammunition for his high-powered rifle was shot dead Monday after he opened fire outside a busy federal courthouse in downtown Dallas, authorities said.

Federal officers killed 22-year-old Brian Isaack Clyde after he approached an entrance of the Earle Cabell Federal Building and began shooting. Glass panes were shattered in a revolving door during the gunfight, but no one else was seriously hurt.

“It’s a lot of rounds — a lot of rounds at his disposal, a large powerful weapon at his disposal,” FBI agent Matthew DeSarno said.

Clyde’s attack began about 8:40 a.m., and three officers from the Federal Protective Service who were stationed at the building confronted him. DeSarno and other officials praised their courage at a news conference.

“But for the actions for the Federal Protective Service officers, this likely would have been a very deadly incident,” DeSarno said.

Officials are still trying to determine a motive, but said there was nothing to indicate the presence of any other shooters or threats to the city, DeSarno said.

A bomb squad examined a vehicle associated with the gunman as a precaution and performed controlled explosions, authorities said. Two loud blasts could be heard.

The Dallas Morning News reported that one of its photographers, Tom Fox, was outside the building and witnessed the shooter open fire. Fox said the masked man got out of his car and began running but stopped to pick up a loaded magazine he dropped on the sidewalk. He then began shooting at the courthouse.

An image of the shooter captured by Fox showed the man wearing a balaclava and a heavy vest and carrying a rifle. Magazines holding 30 rounds each could be seen on his belt.

“I was just praying he couldn’t see me,” Fox said in an interview aired on “NBC Nightly News.”

Another photograph from Fox showed officers tending to a shirtless man lying on the ground in a parking lot outside the building. The gunman was taken to a hospital, where he was pronounced dead, authorities said.