Scotland Yard is closing a long-running investigation into the so-called Croydon cat killer, saying the most likely culprits are foxes.

The Metropolitan Police announced the findings today after reports that a serial cat killer was behind the slaughter and mutilation of up to 500 pets across the UK. For nearly three years, police and animal activists have been hunting for a culprit, also dubbed the M25 cat killer, who was suspected of bludgeoning animals to death before dissecting their corpses.

At one point a team of 15 Scotland Yard officers was deployed on the trail of the suspect, who was even likened to Jack the Ripper.

Animal activists put up a £10,000 reward for his or her capture, while animal-lovers such as actor Martin Clunes urged the police chiefs to step up their search.

Today Scotland Yard announced that a review of all the evidence had found no trace of human involvement, with no CCTV sightings, no witnesses and no forensic leads.

The force revealed that a vet who had carried out post-mortem examinations of six “suspicious cat deaths” had “re-assessed” his findings of human involvement.

Initially, the veterinary pathologist concluded that the cause of death was blunt force trauma, and that the bodies had been dissected with a sharp instrument. Now the same vet has re-examined the bodies and found puncture wounds he had not noticed before, concluding they were likely to have been hit by cars and then scavenged by foxes.

Scotland Yard highlighted similar cases investigated by officers in Hertfordshire, where post-mortem examinations were carried out on three cats and two rabbits in June.

Dr Henny Martineau, head of veterinary forensic pathology at the Royal Veterinary College who assisted the Hertfordshire inquiry, concluded that the mutilations had been caused by “predation and/or scavenging” and said fox DNA had been found around the wounds on all five bodies.

In addition, the police review uncovered three cases of CCTV showing foxes carrying bodies or body parts of cats.

In June last year, a cat’s head was found in a school playground in Catford. CCTV showed a fox carrying it into the playground.

The following month a witness found the body of a cat with no head or tail next to her property. Suspecting that the cat had been placed there, she checked CCTV and saw a fox drop the cat in the position in which it was found.

Police also took note of expert opinion, including a recent New Scientist article, which highlights how wildlife is known to scavenge roadkill, often removing the heads and tails of dead animals.

The Met says it cannot say how much was spent on the investigation, emphasising that all the officers who worked on the case also had other duties. Last year the force admitted that it had funded ten post-mortems on cats at a cost of £7,500.

In a statement the Met said: “Following a thorough examination of the available evidence, officers working alongside experts have concluded that hundreds of reported cat mutilations in Croydon and elsewhere were not carried out by a human and are likely to be the result of predation or scavenging by wildlife.”

An RSPCA spokesman said: “The RSPCA has supported the Metropolitan Police in its investigation into a number of cat deaths over the past three years and we’re pleased that the inquiries have come to a conclusion.”