Approximately 3,000 workers at the Washington Navy Yard were ordered to stay in place this morning after a man armed with an assault rifle, shotgun and handgun opened fire, killing at least 12 people and wounded others, news sources are reporting.

The gunman, who has been identified as Aaron Alexis, was reportedly killed after SWAT officers stormed the building, which is the headquarters of the Naval Sea Systems Command. Alexis, who was originally from Ft. Worth, Texas, recently began working at the facility as a civilian contractor, sources say.

Witnesses said the gunman was wearing dark fatigues and fired at random. The Navy said on its Twitter feed that three shots were fired at 8:20 a.m. EST. Witnesses said the gunman opened fire on the third and fourth floors -- at one point aiming down at people who were in the first floor cafeteria.

President Barack Obama called it a “cowardly act” that targeted “men and women who were going to work, doing their job, protecting all of us.”

The Naval Sea Systems Command is responsible for the manufacture or procurement of submarines and their combat systems.

Schools near the facility were locked down, flights were grounded for a short time at Reagan National Airport and security at the Capitol was tightened. The Department of Transportation was also in lockdown mode during the incident.

Security at the building requires people to scan their ID card and put their bags through a metal detector, but there is no metal detector for the body.

A D.C. police officer and a base security officer were among the reported victims.

Tactical SWAT teams searched an additional building after reports surfaced that there could be a second gunman.