Twelve people were killed and several wounded in a shooting Monday at the US Navy Yard in Washington, where police were searching for two possible accomplices after one gunman died at the scene, dpa reported.
Police said one suspected gunman was among the dead.
"We potentially have two other shooters out there," Washington Police Chief Cathy Lanier said.
Lanier said their involvement was "not confirmed," but that witnesses reported the two men armed at the scene shortly after the shooting began. Both wore "military-style uniforms" - one in drab olive and carrying a rifle, the other in Navy-style khaki with short sleeves and a beret - but were not believed to be members of the military.
"We have no indications of any motive at this time," Lanier said.
Asked if the rampage could have been a terrorist attack, Washington Mayor Vincent Gray said "we don't have any reason to think that at this stage."
There was no information about a motive in the shooting, which began around 8:20 am (1220 GMT) on the sprawling naval base on the Anacostia River, about 2 kilometres south of the US Capitol.
The spree took place inside the Naval Sea Systems Command headquarters, a building where about 3,000 people work. The command is responsible for engineering, building, buying and maintaining vessels and combat systems.
One police officer and two women were being treated in critical condition, Washington Hospital Centre chief medical officer Janis Orlowski said.
One of the women was wounded in the shoulder, the other in the head and hand, and the male police officer was in surgery, she said. "Their chances for survival are very good," Orlowski said.
Hospital trauma teams remained on standby, and authorities "have told us that they believe there will be more victims coming," she said.
Orlowski said that the victims described to medical personnel a perpetrator firing rapidly: "From the reports of the victims, it had to be a semi-automatic."
US President Barack Obama vowed to "stand with the families of those who've been harmed" in what he called "yet another mass shooting."
"These are men and women who were going to work, doing their job protecting all of us," he said. "They're patriots, and they know the dangers of serving abroad, but today they faced the unimaginable violence that they wouldn't have expected here at home."
Obama called for a "seamless" probe of the incident by local and federal authorities: "As this investigation moves forward, we will do everything in our power to make sure whoever carried out this cowardly act is held responsible."
Flights at Ronald Reagan National Airport were halted briefly after the shooting was reported.