An investigation is underway into how 20 swans and ducks ended up dead in a Cambridgeshrie river.
Seven ducks and 13 swans have been found in a two-mile stretch of the Great Ouse in the past fortnight.
It is understood there were no visible signs of injury, and police said: "Tests have proved inconclusive."
A police spokesperson said officers are keeping an open mind but the most likely explanations are deliberate poisoning or a leak into the river.
The affected part of the river, near St Neots Marina, about 20 miles west of Cambridge, runs alongside a golf course and residential streets.
Pc Alun Bradshaw, from Cambridgeshire Police, said: "It is very unusual for such a large amount of birds to die in such a short space of time and we are keen to find out what caused the deaths.
"I would urge anyone with any information to call police."