The zookeeper's kitchen is attached to the apes' enclosure and the zoo says someone simply left the door open Sunday.
That allowed up to six of the western lowland gorillas to escape into the kitchen.
The zoo says two employees have been disciplined, but are still on the job.
A similar incident happened with the kitchen door in March and the employee involved was fired.
Zoo spokesman Lindsey Galloway said staff are reviewing safety protocols to see if they need to be improved, but it may be a matter of people just following the existing rules.
"When all that stands between you and a dangerous animal is a closed locked door, you would think you would have the presence of mind to be sticking with the protocols," he said Wednesday.
"A simple human error with potentially tragic consequences means we can't take such a mistake lightly."
The keeper who was fired earlier this year had previously also taken responsibility for leaving a knife inside the gorilla enclosure in 2009.
Also on HuffPost