A lion-tailed macaque has been stolen from the zoo in the eastern German city of Leipzig.
Unknown perpetrators broke into the enclosure by force, the Leipzig Zoo announced on Tuesday.
The 15-year-old female lion-tailed macaque (Macaca silenus) named Ruma was discovered missing on Easter Sunday during a morning check of the animal enclosure. Her companion, a 12-year-old male macaque, remained in the enclosure and appeared unharmed.
In addition to the "clear signs of a violent break-in," the zoo said it was also apparent that the perpetrators had used traps.
Leipzig police said that detectives had secured evidence of the crime and issued an alert for the macaque.
Zoo director Jörg Junhold said the discovery of the kidnapping was "bad news" and noted that lion-tailed macaque are among the most endangered animal species.
Keeping and transportation are strictly regulated under an international treaty on endangered animals, meaning that the legal procurement of the animals is subject to strict regulations, he said.
Junhold also noted that handling a lion-tailed macaque - also known as a wanderoo - can be dangerous, raising concerns that Ruma may have been injured during her capture.
"We don't know the motive behind the theft, but we are extremely concerned about the welfare of the middle-aged animal, whose species-appropriate keeping and care requires specialist knowledge, and we very much hope that the animal will be recovered or returned unharmed," Junhold said.
As a consequence of the break-in, security precautions at the zoo have been increased, he added.