Outside Fort Worth, Texas, friend Michael Ritrovato said Alexis had recently been frustrated with the civilian contractor about a payment issue. But Ritrovato said his friend never showed signs of aggressiveness or violence, though he played a lot of shooting video games online.
"It's incredible that this is all happening, because he was a very good-natured guy," Ritrovato said. "It seemed like he wanted to get more out of life."
In Seattle, police said they arrested Alexis in 2004 for shooting out the tires of another man's vehicle in what Alexis later told detectives was an anger-fueled "blackout."
SWAT teams swarm area
Meanwhile, at the Navy yard, helicopters hovered overhead. In one chopper, there appeared to be a police sniper peering out, with a scope at the ready.
The Bureau of Alcohol, Tobacco, Firearms and Explosives sent a team of about 20 special agents to the scene, a law enforcement official said. The team was the same group that helped apprehend Boston Marathon bombing suspect Dzhokhar Tsarnaev, the official said.
Emergency personnel, the FBI, U.S. Capitol Police and local D.C. police responded to the shooting, shutting down traffic in the area on the District's south side along the Anacostia River. Some people were evacuated, and others sheltered in place.
Paul Williams, who works at a nearby nonprofit, was headed to his office when he witnessed panic at the Navy yard.
"I heard four rapid bangs -- bang, bang, bang, bang," he said.
At first, he thought it was construction noise, but less than a minute later, he saw hundreds of people coming toward him.
"I didn't know what was happening. I just ran with them," Williams said. "Everyone seemed scared. People were crying. People were being consoled and calling loved ones and family."
Government buildings, schools tighten security
Security was stepped up at the Pentagon.
And at least eight schools were on lockdown as a precaution, the Washington public schools said.
Air traffic to Reagan National Airport in northern Virginia, the closest airport to downtown Washington, was suspended after the shooting but later resumed, the Federal Aviation Administration said.
Officials postponed a Washington Nationals baseball game that had been scheduled for Monday night at Nationals Park, just a few blocks away from the Washington Navy Yard.
The headquarters for Naval Sea Systems Command -- the workplace for about 3,000 people -- is the largest of the Navy's five system commands. It has a fiscal year budget of nearly $30 billion.
"With a force of 60,000 civilian, military and contract support personnel, NAVSEA engineers, builds, buys and maintains the Navy's ships and submarines and their combat systems," the Navy said.
Eleanor Holmes Norton, Washington's congressional delegate, described the Navy yard as a "very secure facility."
The Washington Navy Yard -- the Navy's oldest land establishment -- was created in 1799 following an act of Congress, according to the Naval History and Heritage Command. Originally envisioned as a shipbuilding and fitting facility on the Anacostia River, it serviced some of the Navy's most famous early vessels, including the USS Constitution.