Up to 100 wild swans died in a mysterious lead poisoning which a local vet was convinced was done deliberately.
Wildlife experts, the Environment Agency, the RSPCA and police were involved in an investigation into what was happening to the beautiful birds living on a stretch of the River Wear in Chester-le-Street, County Durham.
There was an outcry when the Press Association broke the story in February, when it was revealed that mute swans were dying in great numbers.
Post mortem examinations showed they had extremely high levels of lead poisoning, but testing of the river showed there was no significant change in pollution levels in the water.
Accidental poisoning from shot or dumped batteries would have shown up in the birds' gizzards as chunks of the metal.
Instead, it was believed the birds were fed balls of bread that had been laced with powdered lead.
The circumstances led vet Gill Maxwell, from Cestria Veterinary Centre, to be sure that a poisoner was the culprit.
She said: "Fortunately, all the publicity seemed to scare off the individual concerned.
"We are totally convinced that this was deliberate poisoning, and had a suspect in mind, but couldn't prove it.
"The lead levels found in the birds and the speed of onset of clinical signs were just totally wrong for an environmental contamination, and, if it had been something catastrophic, there would have been absolutely nothing left alive in the Wear."
In the past six months no more swans have died in suspicious circumstances and numbers have recovered at Chester-le-Street's Riverside Park - an area popular with families to get close to the birds and feed them, she said.
Miss Maxwell added: "Unfortunately, as the levels in affected birds were so very high, most of those referred to a hospital practice for treatment did not actually recover. They seemed to improve initially, but then crashed later on. Very few recovered and were released."
She believed between 80 and 100 died and this will have an impact on the local population as many of the victims were young adults who were yet to breed.