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BERLIN (Reuters) - A fire ripped through the monkey house at a German zoo on New Year’s Eve, killing roughly 30 animals inside, the zoo said on Wednesday.

Chimpanzees, orangutans, a gorilla, bats and birds were among the animals who died at Krefeld Zoo, near the Dutch border. Two chimpanzees, Bally and Limbo, survived, the zoo said.

Describing the fire as an “incomprehensible tragedy”, it said the cause was not known. Local broadcaster WDR said investigators were exploring the possibility it had been started by a celebratory firework.