LATEST: No Shooter Found at San Diego Naval Medical Center
UPDATE 12:39pm ET
Shepard Smith reported that authorities have so far found no evidence of a shooter at Naval Medical Center San Diego, after a single witness reported hearing three gunshots and the center said on Facebook there was a report of an "active shooter" at Building 26.
Lockdowns were lifted at Roosevelt Middle School, San Diego High School and Garfield High School. However, students and staff continue to shelter in place.
But for now, it appears there's no trouble at the facility and that no one was harmed or came under any danger.
A law enforcement official told NBC News that an initial sweep of Building 26 found no sign that any shots were fired.
Fox News Channel is watching the situation closely.
Workers at the Naval Medical Center San Diego have been told to shelter in place because of a possible active shooter, the facility reported.
The center posted that the shooter was believed to be in Building 26. There were no immediate reports of casualties.
The building houses offices and barracks for wounded sailors and Marines as well as a gym. A spokesman said all non-emergency personnel are asked to stay away from the area.
People inside the building are advised to "run, hide or fight."
Witnesses told FOX 5 that the shooter fired several shots into the air and not at any people, perhaps as a warning of some kind.
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.
San Diego High School, Garfield High School and Roosevelt Middle School have all been 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.
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.