SAN FRANCISCO » Employees operating a hydraulic door at the San Francisco Zoo that crushed a baby gorilla were supposed to keep a hand on a stop button at all times, a newspaper reported Wednesday.

The warning was included on a sign near the gorilla door, according to the San Francisco Chronicle which obtained a photo of the sign through a public records request.

It reads: "To prevent injuries to the gorillas during operation of the electric doors: Move only one door at a time. Keep your hand on the stop button at all times. Do not leave the electric panel until operation of the door is complete."

The emergency stop button, similar to those on many garage doors, controls the hydraulic door, but now investigators are looking into whether the zoo employee failed to follow procedure. On Friday the gorilla, a 16-month-old named Kabibe, unexpectedly darted under the door while it was closing.

The employee, in a statement to the zoo, wrote that the accident occurred after Kabibe’s grandmother, Bawang, also her surrogate mother, carried her on her arm into their nighttime enclosure from a public viewing area.

The zoo employee’s name and work history have not been released. It was also not known Wednesday if she is facing disciplinary action.

Animal rights advocates say the stop button could have saved Kabibe.

The zoo has hired animal psychologist Terry Maple, an expert on gorillas, to investigate the death.

The gorilla that was killed is part of a species that is endangered, with only 750 living in captivity and roughly 100,000 living in the wild in Central Africa.

The incident has also renewed concerns about safety at a zoo that has had a history of injuries to animals, visitors and staff members.

On Christmas Day 2007, a Siberian tiger was killed in a hail of police gunfire after leaping and clawing out of her enclosure and fatally mauling a teenager. The cat may have been provoked.