The so-called 'Croydon Cat Killer' is thought to now be taking victims at a rate of around six felines a week, with one animal rescue group saying that the murders have spread across England.

Police and the RSPCA now have around 170 corpses frozen as the killer continues to mutilate more and more animals around the UK. Out of those, only around 35 have undergone post-mortem examinations,with one police officer telling the Croydon Advertiser that each procedure costs around £700: "The borough (on the outskirts of London) can't afford to do that five or six times a week."

Tony Jenkins of South Norwood Animal Rescue and Liberty (SNARL), the group that originally alerted authorities to the supposed serial animal killer, told IBTimes UK they think the killings are the work of one individual as the bodies have all undergone similar mutilation. "Sometimes he just cuts the tail off, sometimes head and tail, sometimes just the head, sometimes it's the head and some paws, sometimes it's literally cut in half, and we've had a few where he's cut the tail and back legs off... there's been one of two where there's been a bit more mutilating... removed internal organs, stripped the carcass almost."

No arrests have yet been made. Police and the RSPCA are now working with SNARL, but Jenkins says that took some convincing. The group started collecting bodies in the autumn of 2015 but it was not until they had around half a dozen carcasses that the police said they had enough evidence to open an investigation.

Post-mortems have been carried out by the RSPCA and by SNARL's own vet. According to Jenkins, both agree that the cats were killed by blunt force trauma before being mutilated by a human. Jenkins says that though they thought they could leave it up to the authorities once they got involved, his group is doing a lot of work – often collecting the bodies and checking the scenes of the crimes. He's worried that if he stops, so will the investigation. Police say that around 10 officers have worked on the case and around 1,200 hours have been spent on the investigation since it was opened on 1 December 2015.

SNARL gets a lot of its information through its Facebook page: "There is a concentration around Croydon and surrounding boroughs which may suggest he lives in that area but we've got killings across the entire M25 area and we've got confirmed killings in Manchester, Birmingham, Northamptonshire, Maidstone and Margate."

A general police statement on the investigation said that some of the dead cats had been found outside of London.

The police say they are "keeping an open mind" about whether the killings are the work of an individual or more than one person. Detective Sergeant Andy Collin, from Croydon CID, said: "We are investigating a number of leads, and are keeping an open mind re suspects, while following up different lines of enquiry. We are now convinced the cats are being killed then mutilated – and would be really keen to hear any relevant information from members of the public who believe they can help us."

Jenkins says that they are advising pet owners to "try and keep your cats indoors at all times, especially at night, and only let them out under supervision where you can keep an eye on them, because this guy can strike anywhere."