Kumbuka, a male silverback gorilla, escaped from his enclosure into a staff area.
Armed police were called, guests were locked down. Zoo officials say nobody was in any danger.
Kumbuka was tranquilized and taken back to his enclosure.
[This is] yet another startling reminder of the risks associated with maintaining dangerous wild animals in captivity.
According to The Guardian, The Born Free Foundation is calling for stricter penalties for zoos that put guests in danger.
The foundation also wants the government to investigate the welfare of great apes in zoos.
CNN and The Guardian contributed to this report.
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