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By Andy Eckardt

MAINZ, Germany — The unexplained death of a Humboldt penguin has prompted police to open an investigation into the zoo animal's grisly demise.

The bird was found outside of its enclosure on Monday after suffering "external injuries." The incident is latest unsolved mystery to hit the attraction in Germany's western city of Dortmund.

Humboldt penguins feeding at their enclosure at the zoo in Dortmund, Germany, on Tuesday.INA FASSBENDER / AFP - Getty Images

In August, five dwarf monkeys were stolen from the zoo's so-called "Amazona" house. A female sea lion was also found dead last month after suffering injuries to its skull.

Officials have revealed that two more of penguins went missing from the zoo on the same night as their companion's death.

"It appears more than unlikely that the penguins escaped their enclosure with a panic jump during the stormy night," zoo director Frank Brandstaetter said.

Katrin Pinetzki, a spokeswoman for the city of Dortmund told NBC News, that there is "no evidence that this was an accident.”

She added that security personnel were now "patrolling the zoo day and night."

Police investigators hope that an ongoing autopsy "will soon provide more evidence in this odd case" and have asked witnesses to come forward.