The mysterious death of a penguin at Germany's Dortmund Zoo has prompted a probe, police said Tuesday, in the latest case of an unexplainable animal death or disappearance at the zoo.
The zoo had informed police on Monday that three Humboldt penguins had gone missing, before later finding one of them "dead with injuries" in the flamingo enclosure.
The death comes just weeks after a sea lion was mysteriously found dead in its enclosure.
The other two penguins remain missing, said police, adding that they were investigating if there had been a "theft of the penguins or whether there had been a violation of the animal welfare act".
At the same time, officers have not ruled out the possibility that the birds simply escaped.
The zoo said it would carry out an autopsy on the penguin on Wednesday, but the latest disappearances came just weeks after a sea lion was found dead in its enclosure.
"This is one in a series of similar incidents this year," the zoo said in a statement.
Three pygmy marmosets -- the world's smallest monkeys -- and two dwarf agoutis -- a type of rodent -- were stolen in August, the zoo said, adding that it had since installed security around the clock.
In early November however, the 21-year-old sea lion was found dead, with a zoo autopsy finding that it had been severely beaten in the head.
The zoo believe that an intruder beat the animal to death, but police have ruled this out.