Update, 10:50 a.m.:
The Navy says an initial inspection at Naval Medical Center San Diego found no sign of a gunman or a shooting.
Navy spokesman N. Scott Sutherland said military police had checked Building 26 after a person reported hearing three shots in the basement earlier in the day.
The base remained on lockdown as authorities went from room to room and led personnel out of the facility.
School officials say a lockdown has ended and instruction has resumed at three schools near the medical center.