At least one shooter was killed following an incident at a Washington navy yard that left several victims dead and several others injured on Monday, a Washington navy official told Reuters.
Earlier authorities said several people were killed and others injured when at least one gunman opened fire at the navy yard.
One navy official said that four people had died and eight others were injured, but other officials suggested caution over those numbers saying the situation was in flux.
A Washington police spokesperson said five people had been shot at the Naval Sea Systems Command headquarters, just 4.1km from the White House, including a District of Columbia police officer and one other law enforcement officer.
"There was three gunshots straight in a row," said Patricia Ward, who works at the navy yard, describing how she first heard the gunfire while having breakfast at the headquarters building.
A few seconds later, Ward said she heard four more gunshots. Security guards rushed in and got people out as fast as they could – "'Run, run, run,' they told people," Ward told reporters.
NBC News, citing a senior naval officer at the navy yard, said at least 12 people had been shot, with four killed and eight wounded. The report could not be independently confirmed.
A navy official, who asked not to be identified, said several people were dead and several others were injured.
Another navy official, speaking on condition of anonymity, said more than one gunman may have been involved. The Washington Post reported that there were several gunmen.
The US Navy said the shooting took place at the heavily guarded headquarters, where about 3 000 people work.
The shooting hit the military establishment less than three week after US Army psychiatrist Major Nidal Hasan was sentenced to death for murdering 13 people in 2009 at Fort Hood, Texas, where he gunned down unarmed soldiers in what he later called retaliation for US wars in Muslim countries.
A Washington police spokesperson said the first reports of shots were received at 8.36am EDT (12.36 GMT).
The White House urged citizens to follow instructions from authorities and first responders, and said President Barack Obama had been briefed.
"The president has been briefed several times about the unfolding situation," a spokesperson said. "The president directed his team to stay in touch with our federal partners, including the Navy and FBI, as well as the local officials."
Shut down flights
Dozens of police and emergency vehicles surrounded the complex in south-east Washington, which is about 1.6km south of the US Capitol, local media reported.
Helicopters circled the headquarters with some touching down on the building's roof, according to a live feed by Washington's WJLA TV. The shooting briefly shut down flights at Reagan National Airport.
The navy yard dates back to the 18th century and is the oldest navy installation. It houses a museum, the residence of the chief of naval operations and is responsible for weapons development, among other functions. – Reuters