LIVE COVERAGE: No casualties, no shooter spotted at Washington Navy Yard

Authorities said on Thursday no injuries were reported at the U.S. Navy Yard in Washington after the facility was placed on lockdown following reports of a possible shooter, according to media reports.

A shooter had not been found as police swarmed the Navy Yard, according to a local CBS radio station, amid heightened security concerns ahead of the U.S. July Fourth holiday weekend.

No further details were available about the incident.

The Navy said the facility was on lockdown but could not confirm any incident.

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 have issued an alert calling for local authorities and the public to remain vigilant for possible threats during the holiday weekend.

The Pentagon ordered new security measures be put in place after a 2013 shooting at the Navy Yard. Aaron Alexis, 34, a former sailor who was working for a government technology contractor at the facility, opened fire on Sept. 16, 2013, killing 12 people. He was shot to death after an hour-long exchange with police.

Alexis had reported hearing voices and being unable to sleep in the weeks leading up to the rampage. The FBI said Alexis believed electromagnetic waves had been controlling him for the months before. He had had prior brushes with the law after firing a gun but the charges against him were dropped.

Watch live video, courtesy of WUSA9, below: