One penguin is dead and two others are missing at the Dortmund Zoo, and police in Germany are investigating a possible string of crimes there.

Police spokesman Marco Mueller said Tuesday the zoo found one Humboldt penguin dead in the flamingo cage on Monday morning and two others gone. He says police are trying to determine if someone stole the birds or if they had somehow escaped.

In November, a 21-year-old sea lion named Holly was found dead in her enclosure at the zoo with a smashed skull and three teeth missing. And in August, three pygmy marmosets and two acouchi rodents vanished.

Mueller says police haven't yet determined if the incidents are linked.

The zoo is adding more security personnel and watch dogs.