Navy security wasn’t able to identify a noise that caused the lock down of Naval Air Station Whidbey Island’s Ault Field base Saturday morning.

Access to the base was shut down and personnel were told to shelter in place after a sailor reported hearing gunfire at about 5 a.m.

Navy Security was concerned about the possibility of a shooter, but nothing suspicious was found, according to Mike Welding, base public affairs officer.

“Out of abundance of caution base officials secured access to and from Ault Field and took all necessary steps to ensure the safety of our personnel,” the Navy press release says.

Welding said security officers completed a thorough investigation but are still trying to determine what the noise might have been.

The noise was heard in an industrial area of the base, according to base officials.

The lock down was lifted just before 10 a.m., but access was still limited. An area was cordoned off as the investigation continued.

The all-clear was given at the base about an hour later.

No evidence of gunfire was located.