WTOP's Dick Uliano says officials on the scene are searching for potentially two other shooters.
Security has been stepped up around D.C., although officials say there is no threat beyond the Navy Yard, according to WTOP's Dave McConnell.
Associated Press: Todd Brundidge, an executive assistant with Navy Sea Systems Command, said he and other co-workers encountered a gunman in a long hallway of their building on the third floor.
The gunman was wearing all blue, he said.
"He just turned and started firing," Brundidge said.
Terrie Durham, an executive assistant with the same agency, said she also saw the gunman firing toward her and Brundidge.
"He aimed high and missed," she said. "He said nothing. As soon as I realized he was shooting, we just said, `Get out of the building.'"
"I've been briefed by my team on the situation," says Barack Obama.
It's a shooting that targeted our military and civilian personnel, says Obama.
WTOP's Megan Cloherty: Three victims in critical condition.
D.C. police chief Cathy Lanier spoke about the shooting and the possibility of two additional shooters:
"We are confronting yet another mass shooting," says President Obama.
WTOP's Dick Uliano: Park Police Chief Teresa Chambers says helicopters are eyes in the sky for officials.
WTOP's Dick Uliano: It's a very dynamic situation and police at the scene are reluctant to give numbers on the shooting.
Witness: 'It was like pow, pow, pow'