The blaze tore through the monkey enclosure shortly before midnight, killing at least 30 animals, police said.
On Thursday afternoon, investigators stated at a press conference that three women – a 60-year-old Krefeld resident and her two adult daughters – had started the fire with three sky lanterns that they had set out for New Year's Eve.
The women turned themselves into the police, said investigators, and stated that they didn't know the lanterns – which they had purchased online – were illegal in Germany.
The enclosure housed gorillas, orangutans, chimpanzees and marmosets. Only two chimpanzees survived, as well as a family of gorillas in a nearby building.
“Our worst fears have been realised,” Krefeld zoo, which specializes in primates, announced on its Facebook page.
Tearful visitors lit candles and left flowers and soft toys at the entrance of the zoo on Wednesday. One of the tributes asked simply “Why?”.
Firefighters prevented the flames from spreading to other buildings at the zoo in North Rhine-Westphalia.
Focus on fireworks
Preliminary findings had already suggested the fire might have been caused by flying paper lanterns, which float into the air when lit.
Three lanterns bearing hand-written New Year's wishes were discovered in the smouldering debris.
These types of devices have been banned in the region since 2009.
Police, who have launched an investigation for “arson through negligence”, were contacted late Wednesday by a number of people who may have used these lanterns. Krefeld police said investigators were verifying their statements.
The German animal protection association quickly called for all kinds of fireworks to be banned near zoos, farms and kennels.
The deadly blaze was “terrible proof of the dramatic consequences for animals” from “uncontrolled” celebrations, the group said.
Germans often use powerful fireworks to celebrate the New Year and in Berlin, rescue services on Wednesday recorded 22 injuries, some of which required amputations, from the holiday.
That was roughly comparable to levels seen in previous years.
The effect of fireworks on air quality has also begun to spark debate and the federal environment agency UBA estimated that the amount of fine particles released in one night was comparable to two months of highway traffic.
Several major German supermarket and hardware chains have decided to stop
Demand remains strong for now however, with the people spending around €113 million for New Year fireworks, the same amount as last year, according to sector federation VPI.
Around 57 percent of the county's inhabitants would support a ban on firework sales but 84 percent of those questioned also said they looked forward to displays next year.
Krefeld zoo planned to remain closed Wednesday with employees “in shock” owing to the “terrible tragedy”, management said.
The zoo has around 1,000 animals and attracts some 400,000 visitors a year.
As of Thursday morning, the zoo remained closed for visitors, it announced on its Facebook page. It has received over 400 donations so far.