The three local women — a mother and her two daughters, ages 30 to 60 — went to police in the western city of Krefeld on New Year's Day after authorities held a news conference about the blaze, criminal police chief Gerd Hoppmann said.
The women are being investigated on suspicion of negligent arson, prosecutor Jens Frobel said. The offense can carry a prison sentence of up to five years.
Hoppmann said the women had ordered five sky lanterns on the internet and told authorities that they had believed they were legal in Germany. He added that there was nothing in the product description showing that they were banned.
Hoppmann described the women as “completely normal people who seemed very sensible, very responsible" and said it was “very courageous” of them to come forward, saving authorities a tricky investigation. He added that they feared reprisals and authorities limited the details given about the suspects.
Investigators believe that just one lantern started the blaze. They found the other four later, with handwritten good wishes for the new year attached.
The destroyed ape house lacked fire detectors and sprinklers, which weren't required when it was built in the 1970s. The zoo said, however, that it had passed a regular fire protection check a few months ago.
The building's roof had been renovated after a hailstorm a few years ago and plexiglass was added, Hoppmann said. He said while investigators were confident the sky lantern was to blame, they will look at other factors that may have contributed to the blaze, such as dry fallen leaves on the roof.
Investigators plan to carry out tests to help find out why the blaze spread so quickly. Firefighters were only able to rescue two chimpanzees. The zoo said Thursday it was satisfied with their condition.