D.C. police said a woman called from inside a Navy Yard building to report that she might have heard sounds of gunshots around 7:20 a.m. However, investigators found no sign of a shooting, a shooter or anyone injured.
No arrests were made and no weapons found, officials said.
PHOTOS: Shooting reported at Washington Navy Yard
"At this time there is no evidence of gunshots," Mayor Muriel Bowser said. "There is no evidence of a shooter, and there is no evidence of any victims today."
A U.S. official who spoke on condition of anonymity 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. Security found no one inside the building, the official said.
Officials do not believe the report was a hoax, D.C. police Chief Cathy Lanier told reporters. Investigators interviewed the woman who made the call, Lanier said, and she did exactly as authorities regularly tell people: Report anything you think may be suspicious.
Shortly after the report, 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. At a news conference, local and Navy officials praised the work of all the responding agencies and called it well-coordinated.
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 have been at the base at the time of the reports, Navy public affairs officer Chris Johnson told reporters outside the Navy Yard, the country's oldest naval installation.
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.
Associated Press writers Brad Foss and Jessica Gresko contributed to this report.