The animals are known to be strong and intelligent
An investigation is under way into how 30 chimpanzees escaped their enclosure at Chester Zoo, forcing its evacuation.
The primates found their way into a nearby keepers' area, where their food is usually prepared, on Sunday afternoon, the zoo said.
Although the area is secure, about 5,000 visitors were asked to leave the zoo site as a security measure.
Zoo director general Gordon McGregor-Reid has apologised for the inconvenience to visitors.
The animal alert went out at 1240 BST after the chimpanzees were discovered in the kitchen area, Chester Zoo said.
How the animals came to leave their quarters is not known but they were not in an area accessed by the public.
The Zoo's blacksmith was visiting the area on Monday to examine the door latches.
Mr McGregor-Reid said: "Somehow or other they got into the kitchen - it must have been the smell of the bananas or something like that."
"It was a bit like an old-fashioned chimps tea party... they've certainly had a ball in that room that's for sure."
The zoo director said chimpanzees got through "lots of food" during their sojourn to the preparation area.
Refunds were offered to visitors after the site evacuation and Mr McGregor-Reid said anyone who had not yet claimed their money back should contact the ticket office.
"All sorts of apologies to our public, we didn't want to inconvenience them but we thought it was far, far better to be safe than sorry," he added.
"The chances of anything really serious happening are vanishingly small but we've got an emergency procedure in the zoo and we follow it.
"Public safety comes first."
The zoo was due to re-open as normal on Monday morning.
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