WASHINGTON - (AP) - The Washington Navy Yard went on lockdown Thursday after reports of shots fired in the same building where a gunman killed 12 workers in a rampage two years ago, but officials said those reports were unconfirmed, with no evidence of a shooting and no weapons found.

No arrests had been made, no weapons were discovered, and no injuries were reported, said the officials, who all had been briefed on the situation or had knowledge of it, but spoke on condition of anonymity because they weren't authorized to release details.

Authorities received an alert about a potential shooter, which triggered a large response in keeping with protocols established after the 2013 massacre, one of the officials said.

Navy security saw surveillance video of two people jumping the fence in the vicinity of the building a couple of minutes before the first report of gunfire, another official. Security found no one inside the building, the official said.

A heavy police and fire department presence began blocks away from the Navy Yard, with roads blocked and a helicopter hovering overhead. The FBI and Bureau of Alcohol, Tobacco, Firearms and Explosives were on the scene.

Gates to the Navy Yard were closed, and all people were advised to shelter in place, said Chatney Auger, spokeswoman for Naval District Washington.

Thousands would be at the base at the time of the reports, Navy public affairs officer Chris Johnson told reporters outside the facility.

In September 2013, military contractor Aaron Alexis killed 12 civilian workers at the Navy Yard's Building 197 before he was fatally shot by police. The building has since been renamed the Humphreys Building. It reopened this year.

When facilities specialist Chris Robertson heard an alarm and loudspeaker instructions about 7:30 a.m., he said his first thought was: "Here we go again."

He said his supervisor called at 7:33 a.m. and told him and his two co-workers to leave. He also said he hadn't noticed anything unusual Thursday morning - everything was normal.

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Associated Press writers Brad Foss, Ted Bridis, Ben Nuckols and Jessica Gresko contributed.