Operations resumed at Kaiser Permanente Fontana Medical Center after it was locked down Thursday night as police responded to reports of an active shooter that were later determined to be unfounded.

Police surrounded Kaiser Permanente Fontana Medical Center, at 9961 Sierra Ave., at 10:30 p.m.

Police combed the hospital for about two hours and did not find an active shooter, according to Fontana Police Department. Around 9:57 p.m., people inside the hospital heard loud noises they thought were gunshots and alerted authorities, police said.

It was not immediately clear what caused the noises, but there was no gunfire at the campus in San Bernardino County, according to police.

Hospital visitors tweeted about the incident. One woman tweeted that she was in a back room with emergency room patients and families who were in waiting room. She said there was no access to a bathroom or water.

Jennifer Silvestri, a Kaiser spokeswoman, said police were at the hospital after reports of an active shooter.

The hospital returned to normal activity just after midnight, police said.