Every animal inside an ape house at a zoo in Germany died after a fire erupted in the early hours of New Year's Day in a blaze that may have been caused by sky lanterns launched to celebrate the holiday, according to officials.

Krefeld Zoo, located near the Dutch border, said on Facebook that its "worst fears have become reality" after the overnight blaze, which killed over 30 animals in total, including five orangutans, two gorillas, a chimpanzee, and several monkeys, as well as fruit bats and birds.

"An unfathomable tragedy hit us shortly after midnight," the zoo said. "Our ape building burned down to the foundation."

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Several witnesses reported that they had seen the cylindrical paper lanterns with little fires inside flying in the night sky shortly after midnight Wednesday near the Krefeld zoo, Gerd Hoppmann, the city's head of criminal police told reporters.

“People reported seeing those sky lanterns flying at low altitude near the zoo and then it started burning,” Hoppmann said.

Police and firefighters received the first emergency calls at 12:38 a.m.

The zoo said the nearby Gorilla Garden exhibit escaped unscathed, with popular resident Kidogo and his family alive and "fine" after the fire.

Only two chimpanzees could be rescued from the flames by firefighters. They suffered burns but are in stable condition, zoo director Wolfgang Dressen told the Associated Press.

“It's close to a miracle that Bally, a 40-year-old female chimpanzee, and Limbo, a younger male, survived this inferno,” Dressen said, adding that many animal handlers were in shock at the devastation.

“We have to seriously work through the mourning process,” Dressen said. “This is an unfathomable tragedy.”

The Krefeld zoo was opened in 1975 and attracts some 400,000 visitors each year. Officials said the zoo will remain closed on Wednesday after the deadly blaze.

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Criminal police officer Hoppmann said that the use of sky lanterns is illegal in Krefeld and most other parts of Germany and asked that whoever launched them or people who witnessed anything to come forward to authorities.

Sky lanterns, which are sometimes also called Chinese lanterns, are a sort of hot-air balloon made of paper. They have been used in Asia for celebratory events for centuries.

Hoppmann said investigators found some used lanterns on the ground that hadn't burned entirely. They were 13.4 inches long, made out of white paper with an opening at the bottom where a small fire would have been suspended. The fire heats the air inside, making them fly and shine at night.

The used lanterns discovered on the ground had handwritten notes on them, Hoppmann said Wednesday.

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Germans usually welcome in the new year with fireworks at midnight. While people are allowed to purchase and launch fireworks, sky lanterns are illegal and uncommon in the country, according to the AP.

The Associated Press contributed to this report.