At least 12 people have been killed and at least four injured in a shooting at a naval installation in Washington DC.
A gunman was killed at the scene. Police were initially searching for two additional "potential" gunman but one has since been ruled out.
Shots were reported at 08:20 local time (12:20 GMT), and authorities have sealed off a swathe of south-east Washington DC.
President Barack Obama said he mourned "yet another mass shooting".
Police responded to reports of two additional suspects in the shooting at Washington Navy Yard, but have since cleared one of them, a white male in a tan short-sleeve military-style outfit.
A black male, 40-50 years of age and wearing olive kit and seen carrying a long gun, was still being sought.
Washington Police Chief Cathy Lanier said 12 people were deceased. Washington Mayor Vincent Gray said four people were wounded in the shooting.
One police officer was shot in the legs and other police officers were injured in other ways.
At least three people including the shot police officer were brought to Washington Hospital Center by helicopter with "severe" gunshot wounds, said chief medical officer Janis Orlowski.
A female civilian was shot in the head and hand, and another woman was being treated for a shoulder wound, she said.
All three were in critical condition but their chances for survival were "very good", Dr Orlowski said.
Authorities said no motive was yet known for the shooting.
The FBI has taken control of the scene and the investigation, Ms Lanier said.
Security around the US Capitol was bolstered following news of the shooting. The US Senate adjourned for the day earlier than scheduled.
The shooting occurred at a building on the Navy Yard campus that serves as the headquarters for the Naval Sea Systems Command, which engineers, purchases, builds and maintains ships and submarines for the Navy.
Eyewitness Patricia Ward reported hearing at least seven shots from inside a cafeteria on Monday morning.
"It just happened so fast… I just ran," she said.
Cmdr Tim Jirus was on the fourth floor of the building when he heard shots, he said.
"It sounded like a cap gun as opposed to a real gun," he said. "I feel very lucky to be alive. Someone standing there talking to me got shot. I didn't."
The Washington DC Metropolitan Police Department has instructed family members to reunite at the car park of a nearby baseball stadium.
President Barack Obama has been briefed on the matter by senior aides and has directed federal agencies to co-ordinate their response and investigation efforts.
At the White House, Mr Obama said he mourned "yet another mass shooting" and sent his thoughts and prayers to the victims.
"These are men and women going to work and doing their job," he said. "They know the dangers of serving abroad, but today they face the unimaginable violence that they wouldn't have expected here at home."
The Washington Navy Yard is the US Navy's oldest shore installation, first opened in the early 19th Century, according to the Navy.