By Ian Simpson
WASHINGTON (Reuters) - Police issued an "all clear" on Thursday after a false alarm about gunshots fired at the U.S. Navy Yard in Washington, the site of a mass shooting that took 12 lives two years ago.
The report about shots and a possible gunman triggered an hours-long lockdown at the military facility, located just a mile (1.6 km) south of the U.S. Capitol.
Thursday's incident came as U.S. law enforcement agencies are on high alert heading into the July Fourth holiday weekend due to concerns about possible attacks inspired by al Qaeda or Islamic State militants."An employee thought they heard something of concern, they made a call," said Chief Cathy Lanier of Washington's Metropolitan Police Department.
"We don't believe that it was a malicious hoax or incident like that," she added.
The phone call was placed from inside the Naval Sea Systems Command Building around 7:29 a.m. (1129 GMT) and was relayed to city police, she said.
Police responded with a massive presence, temporarily shutting down more than eight blocks of nearby roads.
The initial call to authorities reported that shots were heard in the Navy Yard's Building 197, site of the 2013 killings by a worker for a government contractor.
Lieutenant Commander Scott Williams, 39, a guided missile engineer, was in the building on Thursday as well as on the day of the 2013 shooting. Security has been tightened since then, with more ID checks and random bag searches, he said.
He said police came to his office and escorted him and colleagues out swiftly.
"It was pretty much a mirror image of what happened in 2013," he said.
The Navy Yard is on the Anacostia River and houses the Naval Sea Systems Command, along with a human resources operation and a museum and art gallery.