Police have responded to reports of an incident involving an active shooter at a US Navy Yard in Washington, D.C.
The US Navy said in a tweet the Washington Navy Yard is on lockdown and that employees are taking shelter, but no incident has yet been confirmed.
US Park Police, US Marshals and Homeland Security officers quickly arrived at the scene and local roads were closed.
The nearby US Department of Transportation building was also placed on lockdown as a precaution.
There are no immediate reports of casualties.
Federal agencies have been on high alert for terrorist attacks on the US Independence Day holiday weekend.
On Sep. 16, 2013, a number of civilians were killed and four others were injured when a gunman got into a building at the Navy Yard.
The gunman, Aaron Alexis, who was a civilian contractor and former Navy reservist suffering from mental health problems, was killed by police.
#BREAKING: @NavalDistWashington Navy Yard on lockdown/shelter in place. No incident can be confirmed as of yet. More to follow.
— U.S. Navy (@USNavy) 2 Temmuz 2015