Three gunmen have shot more than ten people in an attack at the US Naval Sea Systems Command headquarters in Washington, according to police sources.
The men, one dressed in military fatigues and one carrying an AR-15 rifle, launched their attack on one of the biggest naval command buildings in the US in the early morning rush as people arrived for work.
Authorities have said that several people are dead and at least 10 have been injured.
Police sources told the Washington Post that they were dealing with three gunmen, sparking fears that it could be a terrorist attack. Other sources said one of the gunmen had been "contained".
Six schools in Washington have been put on lockdown.
Authorities have yet to officially confirm the number of attackers.
The President is being regularly briefed on the rapidly unfolding situation as law enforcement teams attempt to locate the gunmen inside the naval base.
Police and federal agents from multiple law enforcement agencies responded as the base, where 3,000 people work, was secured while officers searched for the shooter inside the complex.
Ambulances were parked outside, streets in the area were closed and flights at Reagan National Airport were temporarily halted.
Sky News US Correspondent Greg Milam said: "We see so often in the US, sadly, workplace shooting, a disgruntled employee with an axe to grind who would be behind something like this.
"If it is confirmed there is more than one gunman it suggests a level of coordination, of planing, maybe something more than just domestic issues, maybe you start then to question whether there is a terrorist motivation."
One of the gunmen was said to have been carrying an AR-15 rifle.
The weapon is the country's best selling rifle and the same weapon used in the both the 2012 Aurora cinema shooting where 12 were killed and the Sandy Hook school shooting in Connecticut last year, where gunman Adam Lanza killed 20 children.
A Navy statement said: "An active shooter was reported inside the ... building on the Washington Navy Yard at 8:20 am Eastern Time.
"Emergency personnel remain on scene and a 'shelter in place' order has been issued for Navy Yard personnel.
"This release will be updated as new information is confirmed."
Police from multiple local and federal agencies along with SWAT crews are attending the emergency.
Fox News reporter Peter Doocy said: "It is frantic right now … people are running full steam … all over the place.
"I am looking at what appears to be an all-black armoured vehicle.
"(There are) sirens, ambulances, police cars. Men in full fatigues, with long guns, running down the street.
"Everybody you can think of is here."
A photograph said to have been taken in the yard shows one man lying on the ground and being treated by members of the public.
Naval Sea Systems Command is the largest of the Navy's five system commands and accounts for a quarter of the Navy's entire budget.
It builds, buys and maintains the Navy's ships and submarines and their combat systems.
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