Among the dead was Europe's oldest gorilla, Massa.
A 60-year-old woman and her two daughters are being investigated over a deadly fire that killed more than 30 animals, including Europe's oldest gorilla, Massa.
Officials in Krefeld, Germany, confirmed the women had gone to the police after the fire on 1 January, which investigators believe was caused by paper lanterns released on New Year's Eve.
The women are being investigated for negligent arson, a crime that can carry a prison sentence of up to five years.
Although fireworks are legal and widely used in Germany on New Year's Eve, the use of sky lanterns —- also know as Chinese lanterns — is illegal.
Krefeld Zoo announced on Wednesday that five orangutans, one chimpanzee and two gorillas, as well as smaller species of monkeys and flying foxes, were killed in the fire.
In a press conference, zoo director Wolfgang Dreßen said that two chimpanzees had "miraculously" survived with just minor injuries.
The women bought the five lanterns online, police said, and did not know they were illegal.
Criminal police chief Gerd Hoppmann said that it was “very courageous” of the women to come forward, describing them as “completely normal people who seemed very sensible, very responsible".
Police are limiting the amount of information that they release about the suspects, fearing reprisals.
Meanwhile, questions have been raised about why the ape house had no sprinkler system or fire alarms, prompting the zoo to point out that they were not required when the building was built in the 1970s.
After news of the fire broke early on New Year's Day, Krefeld Zoo confirmed at around 5.30 am that its "worst fears had become a reality".
"An unbelievable tragedy rolled us shortly after midnight. Our monkey house has burned down to the basic scaffolding," it said.
Speaking soon after, German Animal Welfare Society James Brückner commented on early speculation that fireworks may be to blame for the fire.
"If fireworks really caused the fire, this would be a terrible proof of the dramatic consequences uncontrolled fireworks can have," he said.
"At least around areas where many animals live - and of course zoos are among them - there is an urgent need for protective zones where private New Year's Eve fireworks are prohibited."
"Otherwise, even fatal fires cannot be ruled out - regardless of whether they are in zoos, animal shelters or farms. We now hope for a quick clarification of the tragedy in Krefeld for all parties involved."
A video shared on social media showed the blaze as high as the tops of trees as it ripped through the ape house.