An armed shooter stands near the Earle Cabell Federal Building in downtown Dallas Monday. The shooter was hit and killed in an exchange of gunfire with federal officers outside the courthouse. (Tom Fox/The Dallas Morning News)

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 of Fort Worth 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.