The kitchen used by zookeepers is next to the gorilla enclosure at the Calgary Zoo. In March, and again last weekend, zoo keepers left a door unsecured, the Calgary Herald reported.
Six western lowland gorillas made the break Sunday. The zoo said two employees have been disciplined but not fired.
A keeper who left a door open in March was fired. Officials said he had a previous incident -- leaving a knife in the gorilla enclosure in 2009.
Lindsey Galloway, a zoo spokesman, said officials are trying to determine if rules and procedures need to be changed or if it is simply a matter of getting employees to follow existing regulations.
"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. "A simple human error with potentially tragic consequences means we can't take such a mistake lightly."