Tragedy struck a German zoo on New Year’s Eve, when a fire ripped through an ape enclosure and killed more than 30 animals. According to BBC News, police suspect that three women—a mother and two adult daughters—inadvertently caused the blaze by releasing floating lanterns into the sky.

Authorities were alerted to the emergency at Krefeld Zoo, located near the Dutch border, shortly after midnight on Wednesday, reports the Associated Press. The ape house was gutted by flames, leading to the loss of multiple animals, including “highly endangered monkeys like orangutans from Borneo, lowland gorillas from Central Africa and chimpanzees from West Africa,” zoo director Wolfgang Dressen told reporters, according to CNN’s Christian Streib.

Among the dead were 45-year-old Masa, one of the oldest captive gorillas in Europe, and his female partner. Bats and birds were also killed.

It was “the hardest day that Krefeld Zoo has ever had,” said Dressen.

The building reportedly did not have a sprinkler system, and the apes appear to have died from smoke inhalation.

“In death, too, apes are very similar to humans,” noted police investigator Gerd Hoppmann, per BBC News.

Firefighters were able to rescue two chimpanzees, Billy and Lambo, from the ape house. The animals are “only slightly injured,” the zoo said on its Facebook page, and are being cared for by veterinarians. Two chimpanzees and a family of seven gorillas that resided in the neighboring “Gorilla Garden” are also safe, as firefighters were able to prevent the flames from spreading.

Witnesses reported seeing floating lanterns—which are propelled by small fires that make them shine and fly at night—hovering over the zoo on New Year’s Eve. These celebratory objects are illegal in the state of North Rhine-Westphalia, where Krefeld Zoo is located, and across much of Germany due to fire hazard concerns.

In the wake of the devastating incident, three local women came forward and revealed that they had released five lanterns into the sky on New Year’s Eve. The women did not know that such lanterns were banned, authorities said in a statement, and had purchased the items online. Investigators discovered four used lanterns near the zoo’s ape house; the missing fifth one “almost certainly” started the fire.

The suspects are being investigated for negligent arson, which is punishable by a fine or up to five years in prison, according to BBC News. Police noted in the statement that the women are “infinitely sorry” for the damage they may have caused.

Krefeld Zoo said on Facebook that it hopes to rebuild its ape house in the future. But for now, staff are still reeling from the loss of so many treasured animals.

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