FRANKFURT, Germany -- German police are hunting a killer responsible for the death of 15 flamingos at Frankfurt Zoo, some of which were beheaded as they slept.

The culprit, or culprits, struck twice, both times at night.

Keepers found nine of the long-necked, pink birds dead in their enclosure on Friday, and another six Saturday.

Zoo director Manfred Niekisch says necropsies were being conducted Monday to determine how the Chilean Flamingos were killed.

He says the killer could have been human, but bite marks indicated a fox might also have attacked the animals, though they may have already been dead when that happened.

Niekisch says despite putting in place extra security after the first incident guards weren't able to prevent the second attack.

He says flamingos don't normally cry for help when attacked.