First responders and Navy units with dogs searched the building where the shots were reported and found no evidence of shooting nor anyone injured or killed, according to Cmdr. William Marks, a Navy spokesman at the Pentagon. Another floor-by-floor inspection followed, and the shelter in place order was not fully lifted at the facility until mid-afternoon, the Navy said.

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The medical center posted on its Facebook page shortly after 8 a.m. local time that an active shooter had been reported in a building at the facility.

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“All occupants are advised to run, hide or fight,” the message stated.

The Navy said in a statement a little more than an hour later that according to a single witness, “three shots were reportedly heard” from the basement of Building 26, which houses barracks and a gym. Officials responded to the scene to clear Building 26, the statement said. As a result, the medical center remained locked down and a “shelter-in-place” order was given.

By Tuesday afternoon, with the shelter-in-place orders lifted, it was still not clear what prompted the report. The lockdown came at a period of heightened tension nationwide, occurring a little less than two months after 14 people were shot and killed in what authorities called a terrorist attack in San Bernardino, Calif., about an hour north of the San Diego facility. Last summer, a gunman killed four Marines and a Navy petty officer at two military facilities in Tennessee.

Military police contacted the San Diego Police Department shortly after 8 a.m., saying that shots were fired at the installation and no assistance was needed, said Officer Travis Easter, a spokesman for the city’s police force. The San Diego Fire-Rescue Department and the San County Sheriff’s Department also said they were not asked for help.

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The San Diego Unified School District announced that it had locked down two high schools and a middle schools in the area as a precaution, but that was lifted at around 9:30 a.m. Pacific time. Students and staff at those three schools continued to shelter in place for a short time, but that was also lifted, the district said.

The medical center has a staff of more than 6,500 people, including military, civilians, contractors and volunteers, according to its site. It spans 1.2 million square feet in the city’s Balboa Park, and on an average day there are thousands of patients visiting for medical or dental treatment.

The hospital grounds also were locked down last July after as police searched for a suspect in an armed robbery. In that case, the suspect was not found on the hospital campus and was eventually taken into custody.

In December 2012, the Navy posted video of an active shooter exercise taking place at the San Diego facility, a little more than nine months before a gunman killed 12 people and wounded others during a shooting spree at the Navy Yard installation in Washington.

Washington’s Navy Yard was locked down for three hours this past July after a woman reported hearing gunshots, but a sweep of the building eventually found no evidence that shots were fired.

Lindsey Bever and Peter Holley contributed to this report.