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By Sky News US Team
Reports have said an "active gunman" has fired shots at the Navy Yard in Washington DC, site of a mass shooting in 2013.
Video footage showed armed US Marshals wearing body armour outside the gates of the defence facility on the Anacostia River after it was placed on full lockdown following reports of shots being fired.
A large number of police vehicles were at the scene, with the gates to the yard shut and no one allowed to enter.
The alert is focused on building 197 on the base. In September 2013, military contractor Aaron Alexis killed 12 civilian workers after he went on a shooting rampage at building 197, before he himself was shot dead.
An official has said there are no reports of injuries and no shooter has been identified.
A tweet from the US Navy account said: "Navy Yard on lockdown/shelter in place. No incident can be confirmed as of yet. More to follow."
The pedestrian plaza on Pennsylvania Avenue, in front of the White House, has been sealed off by law enforcement after the Navy Yard alert.
The Navy Yard, in the southeast of Washington DC, is the country's oldest naval installation.
Building 197, which has been renamed the Humphreys Building after the 2013 shootings, has had new security measures put in place by the Pentagon and reopened this year.
It comes amid heightened security in the US ahead of Independence Day celebrations on Saturday.