The civilian employee who reported three shots fired at the San Diego Naval Medical Center on Tuesday genuinely thought that's what she had heard, officials said.
The Navy on Wednesday said it's doing a "hot wash" -- military parlance for looking at lessons learned -- after the incident, which sparked a massive "Code White" active-shooter lockdown throughout Tuesday morning until it was determined to be a false alarm.
Hospital spokesman Mike Alvarez said no formal investigation will be conducted because there's no suspected wrongdoing.
Sailors, Marines and civilians working at the hospital were instructed to "run, hide or fight" -- the advice given on the Department of Homeland Security's "pocket card" issued last year and used in military training.
But as a practical matter, any fighting would likely have been hand-to-hand combat, because the majority of troops on American bases are not armed.
Defense Department guidelines going back to at least 2011 control who carries weapons on bases, restricting it mostly to military police and federal civilian law enforcement.
So at San Diego Naval Base at 32nd Street, for instance, basically the only armed personnel are master-at-arms sailors guarding gates or conducting patrol, Navy civilian police and sailors stationed at each ship's entrance and watch-stander positions.
The Navy hospital, situated in Balboa Park, follows the same rules.
The policy became a national issue last summer, after the July attack on a military recruiting station and a Navy reserve center in Chattanooga, Tenn.
A Navy officer returned fire with his personal weapon as Muhammad Youssef Abdulazeez shot at troops at the reserve center. One of the Marines killed was also armed.
There was speculation, and some outrage, about whether the Navy officer, Lt. Cmdr. Timothy White would be charged for carrying the gun against Defense Department policy. He hasn't been charged, a Navy official confirmed Wednesday.
After the July attack, Congress tinkered with legislating a looser policy, but the Pentagon came out against the idea.
That meant sailors, Marines and civilians at the Navy hospital's wounded warrior barracks and recreation center, known as Building 26, probably didn't carry weapons Tuesday.
Many of them evacuated. Some locked their doors and hid inside, as instructed.
Homeland Security's active-shooter advice card tells people to fight only as a last resort. It suggests trying to incapacitate the shooter by "acting with physical aggression" or throwing things.
The civilian who reported hearing shots from the Building 26 basement found a Navy master-at-arms and sounded the alert at 8 a.m., just as the hospital was spinning to life for the day.
The hospital campus is secured by cameras and locks, but officials wouldn't discuss details about those security measures. Navy hospitals across the country have similar procedures.