(NBC) -- A man brandishing an assault rifle, shotgun and handgun opened fire Monday inside a building at the Washington Navy Yard — killing at least four people and wounding at least seven more, including two police officers, authorities said.
SWAT officers swarmed the building, the headquarters of the Naval Sea Systems Command, and a shooter there was killed, sources told NBC News. More than two hours after the first gunfire, there were reports that tactical teams were entering a second building at the Navy Yard.
The Washington police chief said there could be as many as two other shooters at large. She said officers were on the lookout for two men, one white and one black, both in military-style uniforms.
Terrie Durham, who works at the Naval Sea Systems Command building, said the gunman appeared to be wearing dark fatigues. Another worker there, Todd Brundidge, said he heard a fire alarm go off, and later saw the gunman come around the corner.
"He turned our way and started firing, and we ran downstairs to get out of the building," Brundidge said. "No words. He raised the gun and started firing."
A naval security guard was among those shot and was hit in both legs, U.S. military officials said. Washington city police told WRC, the NBC affiliate in Washington, that one of their officers was also among those shot. It was not clear how many of the others shot were civilian and how many were military.
Patricia Ward, who works at the Navy Yard, said she had just gotten breakfast in the cafeteria when she heard "three gunshots, pow-pow-pow, straight in a row."
"All of the people that were in the cafeteria, we all panicked, and we were trying to decide which way we were going to run out," she said. "I just ran."
Tim Hogan, a spokesman for Rep. Steven Horsford of Nevada, posted photos to his Twitter account of people tending to at least one person down on a street corner.
Chaos enveloped the surrounding neighborhood for hours. Flights were briefly grounded at Reagan National Airport, and nearby schools and the headquarters of the Department of Transportation were locked down. Farther away, police stepped up security on the Capitol grounds.
Dr. Janis Orlowski, the chief medical officer at MedStar Washington Hospital Center, said that the hospital was treating three gunshot victims — a woman hit in the head and hand, a woman hit in the shoulder and a man hit in the legs.
She said that the victims came in alert and talking.
"They're talking about gunshots that they heard in rapid succession," she said.
George Washington University Hospital said it had one patient.
The Navy said on its Twitter feed that three shots were fired at 8:20 a.m. ET at the Sea Systems Command headquarters. About 3,000 people work there, the Navy said. They were ordered to stay in place. WRC video showed a medical helicopter lifting someone off a roof.
"This is a huge piece of land with several building so it's going to take some time for us to get through it and search it so we can make sure it's safe," Peter Newsham, assistant chief of the Washington police, told reporters.
The White House said President Barack Obama was getting regular briefings.
The Naval Sea Systems Command builds, buys and maintains ships and submarines and their combat systems. The Navy Yard is along the Anacostia River in Washington, near the Washington Nationals baseball stadium.