UPDATE: (NBC News) - Police and ambulances swarmed to the Washington Navy Yard on Thursday after a report of gunshots at the campus, where a gunman killed 12 people in a rampage two years ago. Authorities gave the all-clear about two hours later.
A security sweep of the Navy Yard buildings found no evidence of a gunman, and it was not even clear where the reports of gunshots came from.
The police response began before 8 a.m. Workers on the campus were ordered to stay where they were, and workers who had not reported to work were told to stay home.
"It was pretty much like the first time, a lot of people running for the exits," Navy Lt. Cmdr. Scott Williams said.
Facebook Twitter Google PlusEmbed Navy Yard Employee Describes Chaos Amid Reports of Active Shooter 1:02 Local and federal authorities planned a press conference to "calm everyone down," a U.S. official said.
The police response was extraordinary, both because of the 2013 attack and because of warnings by law enforcement about ISIS-inspired attacks around the July Fourth holiday.
Washington police, the FBI, the Department of Homeland Security and federal marshals all rushed to the scene after the reports came in. Washington police told standby units not to respond.
Nearby roads were closed and nearby buildings evacuated. Security was stepped up at the White House, a law enforcement official said.
On Sept. 16, 2013, Aaron Alexis, a former Navy reservist with a troubled mental health history, opened fire in an office building at the Navy Yard, paralyzing the capital before he was taken out in a gunfight with authorities.
Alexis was in the building for more than an hour before he was stopped. It was the second-deadliest rampage on an American military base, after the Fort Hood shootings in Texas in 2009. The building where Alexis opened fire reopened in February after renovations.
The Navy Yard is in southeast Washington, near the headquarters of the Transportation Department and the ballpark of the Washington Nationals. It is about two miles from the Capitol.