Washington: A US Navy installation in Washington was given the all-clear Thursday after locking down for several hours over unconfirmed reports that an active gunman was on the loose, authorities said.
The Navy said on Twitter that there was "no sign of shooting" and that all personnel were fine.
Local media reported that the district police and the FBI also had given a thumbs-up after a building-to-building search of the US Navy Yard turned up no evidence of a shooter or that any shots were fired.
The Navy Yard, a complex of naval offices in southeast Washington, was the scene of a September 2013 mass killing in which a lone gunman fatally shot 12 people before being killed himself by police.
Thursday`s lockdown also came amid security jitters as the United States braces for the July 4 Independence Day holiday, with major festivities planned in the capital.
The FBI, Department of Homeland Security and National Counterterrorism Center have warned the public of an increased risk of attack during the long Independence Day weekend.
The Navy Yard was locked down at around 7:45 am (1145 GMT) on the reports of an active gunman on the premises.
Military and civilian workers streamed out of the Navy Yard, seen in television images with hands in the air, after receiving orders to evacuate or take shelter in place.
"We heard someone scream, `Get out of the building, stay away from the cafeteria` and we saw everyone running for the exits or adjoining offices," Lieutenant Commander Scott Williams said on CNN.
But, like other witnesses, he said he heard no shots or saw any signs of a struggle.
A couple of dozen troops in fatigues were seen assembling on a nearby soccer field.
Todd Parker, who worked on the fifth floor of the building where the 2013 shootings unfolded, said he and his colleagues locked themselves in their office.
"About a half hour later we heard police, law enforcement coming through, banged on the door, `Police, open up,` guns drawn, hands up. A lot of law enforcement (were) in the building escorting us out," he told CNN.
Police cars converged on the scene in southeast Washington, and a huge traffic jam formed outside the complex as two helicopters and a fixed wing aircraft circled overhead.
During the lockdown, authorities stepped up security outside the White House, closing the pedestrian mall that runs along Pennsylvania Avenue and the adjoining Lafayette Square.
The Navy sent out a stream of Twitter messages throughout the incident, advising people to "shelter in place" while stressing no "incident can be confirmed as of yet."