A large police presence gathers along M St. in Southeast Washington, Thursday, July 2, 2015, after an official said shots have been reported in a building on the Washington Navy Yard. [Photo/IC]

"It was pretty much a mirror image of what happened in 2013," he said. "Here we go again."

Williams said security had been tightened after the 2013 incident with more ID checks and random bag searches.

Live television showed a massive police presence in the area, and more than eight blocks of nearby roads were shut down as authorities responded to the scene, the Metropolitan Police Department said.

The Department of Homeland Security and the Federal Bureau of Investigation have issued an alert calling for local authorities and the public to remain vigilant for possible threats during the holiday weekend.

The Pentagon ordered new security measures be put in place after the 2013 shooting. Alexis, 34, had opened fire on Sept. 16, 2013, killing 12 people. He was shot to death after an hour-long exchange with police.

Alexis had reported hearing voices and being unable to sleep in the weeks leading up to the rampage.

The Navy Yard, on the Anacostia River, was established in 1799 as a shipbuilding site and a base for defending the city. The complex houses the Naval Sea Systems Command.