WASHINGTON — As many as two gunmen launched an attack Monday morning inside the Washington Navy Yard, killing at least 12.

They sprayed gunfire on office workers in the cafeteria and in the hallways at the heavily secured military installation in the heart of the nation’s capital, authorities said.

One gunman was dead after he fired on a police officer, and police hunted for a second possible attacker who may have been disguised in a military-style uniform, authorities said. It wasn’t clear how the gunman died.

Investigators said they had not established a motive for the shooting rampage, which unfolded less than four miles from the White House.

As for whether it may have been a terrorist attack, Mayor Vincent Gray said: “We don’t have any reason to think that at this stage.”

The dead gunman is identified as Aaron Alexis, 34, from Texas.

He is believed to have gotten into the Navy Yard by using someone else’s identification card. It is not yet clear if that person was an accomplice or if the ID was stolen.

The Washington Nationals cancel their Monday night game with Atlanta Braves due to the shooting.

Senate Majority Leader Harry Reid, D-Nev., announced that the shootings a little over a mile south of the Capitol prompted him to adjourn the Senate and postpone votes until Tuesday.

The House side of the Capitol complex, which is closer to the Navy Yard, remained open.