Further tests are being carried out after 10 dead birds were found by the Grand Union Canal in Buckinghamshire.
One mute swan, two mallards and seven moorhens were found on a disused stretch from Wharf Road in Wendover to Brookside in Halton.
Police are investigating and said a pellet was found in one bird.
Tiggywinkles Wildlife Hospital, which helped retrieve the bodies found on Saturday, said it did not look like a freak accident.
No pollutants
Clare Campbell, of the Haddenham-based hospital, said the bodies were spread over "the best part of a mile".
"The swan was from a family unit that the public know well," she said.
"There was no sign of any pollutants so it did not look like a freak incident.
"The area is well used by the public so I hope we get closure by finding out who has done it from a human point of view as well."
Thames Valley Police wildlife crime officer, PC Dean Kingham, said the area was popular with walkers and urged anyone with information to come forward.
