Gorilla made opportunistic escape through unlocked doors at zoo
The silverback gorilla whose escape sent ZSL London Zoo into lockdown made an "opportunistic" exit through two unlocked doors into a corridor where a keeper was working, the zoo said.
Professor David Field, the ZSL's zoological director, said Kumbuka was kept calm by the member of staff, with whom he had a "close bond".
Armed police were called to the central London attraction and visitors were evacuated when the alarm was raised following the ape's bid for freedom on October 13.
The incursion into the zookeeper area culminated in Kumbuka glugging five litres of undiluted blackcurrant squash, before being tranquillised and moved back to safety, an in-house investigation into the incident said.
Despite the animal's security breach, Professor Field said the whole affair was "less dramatic than some would have you believe".
He added: "I can certainly tell you that there were no broken locks, Kumbuka did not smash any windows, he was never 'on the loose', and his normal gorilla posturing reported by visitors earlier in the day was unrelated to the incident."
Kumbuka's daring escape began after he was called into his enclosure for dinner shortly after 5pm.
The 29st "alpha male" of the zoo's troop then found the door to his area was unlocked and a second door had yet to be secured, leading him to come face-to-face with the zookeeper.
Professor Field wrote: "Thanks to the incredibly close bond and relationship shared by the zookeeper and Kumbuka, the zookeeper was able to continually reassure Kumbuka, talking to him calmly and in the same light-hearted tone he would always use, as he removed himself from the area.
"Staff raised the alarm that triggered our standard escape response, while Kumbuka briefly explored the zookeeper area next door to his den, where he opened and drank five litres of undiluted blackcurrant squash.
"Kumbuka was immediately contained in the non-public area by quick-thinking zookeepers responding to the alarm, where he was tranquilised and moved back into his den."
He added that the human error which facilitated the escape was rare and the risk of mechanical failure meant having an automated security system posed a "greater" threat.