WASHINGTON -- Police on Thursday responded to a report of a shooter at the US Navy Yard in Washington, where a gunman killed 12 people in 2013, and the military installation was on lockdown with employees sheltered in place.

No further details were available about the incident, which comes amid heightened security concerns ahead of the US July Fourth holiday weekend.

A police officer and local media said authorities responded to a report of an active shooter at the facility.

"No incident can be confirmed as of yet," the Navy said. "Investigation continues."

FOX5, the Fox News television affiliate in Washington, reported a shooting at the facility just after 7:30 a.m. EDT (1130 GMT), citing multiple sources. NBC4, the local TV affiliate, said authorities were going building by building and room to room searching the compound. Reuters could not immediately verify the reports.

Live television showed a massive police presence in the area, and more than eight blocks of nearby roads were shut down as authorities responded to the scene, the Metropolitan Police Department said.

The Department of Homeland Security and the Federal Bureau of Investigation had issued an alert calling for local authorities and the public to remain vigilant for possible threats during the holiday weekend.

Aaron Alexis, a 34-year-old government technology contractor at the Navy Yard, went to the facility on Sept. 16, 2013, opened fire, killing 12 people. He was shot to death after an hour-long exchange with police.

The FBI said Alexis believed electromagnetic waves had been controlling him for months before the rampage.