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SAN DIEGO – Workers at the Naval Medical Center San Diego were told Tuesday morning to shelter in place because of a possible active shooter, the facility reported.

According to a written statement from the U.S. Navy, a single witness reported hearing three gunshots in the basement of Building 26 at about 8 a.m. That building is a combination gym and barracks at the medical center, the Navy said.

First responders and Navy dog units were searching Building 26 and the medical center has been locked down, with all personnel ordered to shelter in place, the Navy said. There were no immediate reports of casualties. All gates to the medical center have been closed.

#BREAKING #navalmedicalcenter Large law enforcement presence, hearing doctors & nurses cleared to go back inside. pic.twitter.com/uryj2MPugf — Sharon Chen (@SharonChenTV) January 26, 2016

As of 10:10 a.m., Navy personnel have completed an initial sweep of Building 26, according to Kevin Dixon, public information officer of Navy Region Southwest. Searches found no evidence of a shooting anywhere in the building, he said. Searchers were in the process of a second search to be thorough, but so far no bullets, casings or bullet holes have been found, Dixon said.

Witnesses told FOX 5 reporter Robert Burns that the shooter fired several shots into the air and not at any people, perhaps as a warning of some kind.

A construction worker at the hospital spoke with FOX 5 reporter Jaime Chambers by telephone and said that he was sheltering in place at the facility. He said he had heard several gunshots and seen heavily armed law enforcement officers searching the facility.

San Diego High School, Garfield High School and Roosevelt Middle School were all locked down as a precaution because of their proximity to Naval Medical Center San Diego, said Linda Zintz, a spokeswoman for San Diego Unified Schools. "Parents should be reassured that their kids are safe and we are monitoring the situation closely," Zintz said.

As of 9:30 a.m., the lock-downs had been lifted at all three schools, according to a tweet from the school district.

All Navy childcare facilities have been concerned, according to a Facebook post by Naval Base San Diego post.

Parents should stay at home so as not to create a chaotic situation outside the schools.

U.S. Marshals and the California Highway Patrol were assisting at the hospital. The San Diego Police Department told FOX 5 that they had received reports of a shooter at the hospital, but their assistance had not been requested.

This developing story will be updated as more information becomes available.

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