San Diego (AFP) - Police rushed into a military hospital in San Diego on Tuesday after reports of gunfire, but an initial sweep of the sprawling facility found no sign of a shooting.
A single witness reported hearing three shots in the basement of one of several buildings in the Naval Medical Center complex in southern California, prompting a massive response from authorities.
Local police, assisted by Navy dogs and California Highway Patrol officers, scoured the building after the call came in around 8:00 am (1600 GMT) but there was no sign of a shooting or any reports of casualties, officials said.
"They did an initial sweep and they didn't find anything so they are doing a more thorough sweep," Navy spokesman Jon Nylander told AFP.
"They have not located any evidence of a shooting having taken place."
Authorities placed the hospital and surrounding buildings on lockdown and local news footage showed patients and workers in hospital garb -- as well as service members in military dress -- filing out of the facility with their hands raised as police stood by with assault weapons at the ready.
US law enforcement officers routinely train for "active shooter" scenarios in which an armed person goes on a rampage.
Such events have become all too common in America, still shaken after last month's mass shooting in San Bernardino, also in southern California.
In that case, a married couple inspired by Islamic State jihadists slaughtered 14 people in the city east of Los Angeles.
The Naval Medical Center was quick to get word out of a possible shooting, posting a warning on Facebook.
"An active shooter has just been reported in building #26 at Naval Medical Center San Diego. All occupants are advised to run, hide or fight," the hospital posted on its official Facebook page.
San Diego, the southernmost city on the California coast, has a sprawling military infrastructure and is a major port for the US Navy.
The hospital is located in the city's Balboa Park, close to San Diego's famous zoo.
The United States has seen several deadly shootings at military installations in recent years.
In November 2009, US Army psychiatrist Major Nidal Hasan opened fire at his Texas military base, Fort Hood. He killed 13 people and wounded more than 30 more, before being overpowered by police.
In September 2013, Aaron Alexis killed 12 people and wounded eight others at the Washington Navy Yard, just two miles (three kilometers) from the US Capitol building, before he was shot dead by officers.