The M25 cat killer is believed to have slaughtered around 400 cats and small animals, leaving their bodies in plain sight in order to "horrify" people.
Police are hunting the killer who has been described as a "psychopath", and kills the animals with a blunt force, before mutilating them with a sharp implement.
The attacker was first dubbed the Croydon cat killer, because it is believed the killings started in the south London area in around October 2015.
However, in the last three years animals in Surrey, Manchester, Sheffield and Birmingham are thought to have been killed by the culprit.
South Norwood Animal Rescue and Liberty (Snarl) has been cataloguing the deaths, and helping police with the investigation.
Snarl co-founder Tony Jenkins said: "We are seeing the exact same injuries, and he leaves a signature.
"If there is more than one killer, it is likely to be a joint enterprise rather than a copycat killer.
"We suspect he gets off on hanging about and watching people's reactions. We can't prove that, but we strongly suspect it is the case because of the way he is displaying the bodies.
"It is definitely an act against humans on that basis - he leaves them underneath bedroom windows whilst he hangs around to see someone find their beloved cat decapitated."
Asked what he thought the killer's motive could be, Mr Jenkins replied: "I think the motive is to horrify humans. Why else would you get a fox and cut its head completely off and then place the body pointing to the children's playground not 10 metres away.
"It is definitely an attack on humans, to terrify humans."
He added that Snarl believed that up to 400 animals, mainly cats, but also including rabbits and foxes, had been decapitated and some had had their tails severed by the attacker.
The Metropolitan Police has been working with the RSPCA and Snarl.
To date there have been no arrests, and no specific suspects identified, though officers continue to pursue a number of leads.
Scotland Yard said there is no evidence that there is a risk to human life.
A £10,000 reward has been offered to anyone who had information that could lead to the arrest of the killer.
Detective Sergeant Andy Collin, from Croydon CID, said: "We are investigating a number of leads, and are keeping an open mind regarding 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."
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