The sky is thick with grey cloud. A nearby road drones drearily with the noise of traffic, and pupils swing their rucksacks around on the way home from school in Croydon, South London. But just out of sight from the main road, the lifeless body of a cat is lying in a pile of leaf litter. Tony Jenkins has seen many like it before.

He slaps on a blue vinyl glove. "I've got the chip checker," he calls to his partner Boudicca Rising. They have already had a look at the body, which is lying stretched out behind some iron railings.

"We're assuming it's been run over and the body's been dumped there, but it's missing some skin," says Rising. It could be a sign that the cat has been tampered with. She coughs and keeps her arms crossed to ward off the cold. "Welcome to our life," says Jenkins with a laugh.

Jenkins and Rising run South Norwood Animal Rescue and Liberty (Snarl). Since late 2015, they have been on the trail of what appears to be one of the most prolific domestic animal killers ever recorded. Investigators have found more than 100 mutilated cats, often with their head or other body parts severed.

Those on the case think it is the work of the same individual. He or she may be killing the cats, or simply finding their bodies and then dismembering them. But either way, no-one seems able to cite another chain of crimes quite like it in Britain.

"Paw looks bust," notes Rising as Jenkins, now bent over the body, handles the limb with care. "He's hit her right full on in the face," he says, examining the injuries. "Poor little soul."

This looks like a bad collision with a road vehicle, the pair agree – not likely the work of the killer. But they take their time and treat every call-out as a potential incident of interest until it has been examined. This one was a tip-off from a local resident who found the body.

The cats' remains are often left in visible places, even directly below house windows

Since bursting into the national headlines in December 2015, stories about a cat killer mutilating pets in South London and further afield – even as far away as Birmingham or Manchester – have shocked the UK. A handful of celebrities have thrown their weight behind efforts to catch the perpetrator. Also, a reward of £10,000 for information leading to his or her arrest and conviction is being offered by animal charities People for the Ethical Treatment of Animals (Peta) and Outpaced.

Rising struggles for a moment to steady a smartphone as she takes a photograph of the identification number that has flashed up on the chip checker's screen. The number has been detected on a chip embedded in the cat's body. This means the owner can be found and the cat returned for burial or cremation.

For now, though, Rising and Jenkins must take ownership of it. As carefully and respectfully as possible given the circumstances, they lift the body into a black bin bag before putting it into the boot of their car. "It's a good job, really, we found her before the owners did," says Rising.

It began in September 2015. A local pet group posted a Facebook update about cats being found dead and with a variety of mutilations. It caught Jenkins's attention. He shared it on Snarl's own Facebook page and started making enquiries.

Many people they initially spoke to dismissed the killings as the likely work of foxes

"Shortly after that, we found out that a cat that had gone missing on the Tuesday had actually been found on the Wednesday morning 20 doors down the road, mutilated and with no blood, on the front path near the doorstep," remembers Jenkins. They would later realise that this was a characteristic deposit. The cats' remains are often left in visible places, even directly below house windows, seemingly to create a display that shocks.

However, this body had been quickly removed by the council. There was no way for Rising and Jenkins to assess what kind of injuries had been sustained.

But a few weeks later in October, after a warning to residents had been published in the local press, Snarl got a call from a pet owner. His cat, Amber, had been found decapitated like the last victim.

"He knew the way the head had been cut off was too clean, he knew it wasn't right," recalls Jenkins. "He remembered seeing the article in the paper and he rang us. That was our first body."

There had been cases of cat killings and disappearances in the same area in the past, but nothing that had these distinctive features. Jenkins and Rising were instantly convinced that the case needed more attention. But at first they say they were frustrated by efforts to engage the police and the Royal Society for the Prevention of Cruelty to Animals (RSPCA).

Many people they initially spoke to dismissed the killings as the likely work of foxes, which are common all over London. Amber's owner instinctively felt that explanation did not fit. Jenkins and Rising agreed. "We realised that nobody was going to do anything," says Rising. The investigation became their own.

Animal abuse is sometimes a precursor to attacks on people

With a body now in their possession, and support from Amber's owner, Jenkins and Rising enlisted the help of their vet, who had forensic training and could perform a post-mortem examination.

"He was horrified by what he saw," remembers Jenkins. "He said absolutely no way was this a fox, unless the fox was carrying a knife."

"And no saliva," adds Rising. The vet was convinced the cat had been mutilated by a blade gripped by a human hand. There was no other logical explanation.

With a report spelling out these details, it was easier to get the attention of local police. Information on the killings and the post-mortem eventually made its way to the desk of Det Sgt Andrew Collin, from the Metropolitan Police in Croydon. Collin decided to take the case seriously. After all, if someone was going about the streets of the capital with a knife, attacking and mutilating cats, there was a chance he could be dangerous to members of the public as well.

Since the 1960s, detectives have been aware of a disturbing pattern: animal abuse is sometimes a precursor to attacks on people. A 2004 study found that repeated childhood animal cruelty was a predictor of recurrent acts of violence towards humans.

Collin needed confirmation that the vet's assessment was credible. He enlisted the help of an RSPCA investigator, Mike Butcher.

Butcher admits that, at the time, the RSPCA was not convinced that the dismemberments were being carried out by the same person. But the RSPCA arranged its own post-mortem examinations and they came to the same conclusion. "We realised they were right in as much as there was something going on, when we found out one person was doing it," says Butcher.

While the police currently have a list of "persons of interest", there is no identified suspect

The total of post-mortem examinations now stands at nearly 40. All are in agreement: a human interfered with each cat. Plus, that is only part of an estimated 170 bodies that have been collected, either by Snarl or the RSPCA, over the past year.

"I've not had one like this before in all the time I've been in the RSPCA," says Butcher. "Its magnitude is unprecedented. He needs to be caught – he will be caught."

Besides the high body count, those on the investigation have been surprised by the wide geographical area that has been covered by the killer. At first the cases were confined to London, but then Snarl started picking up reports of unnervingly similar incidents in other towns. These have included Maidstone in Kent, Birmingham in the Midlands and even Manchester and Sheffield in the north.

"I'm treating them as linked," says Det Sgt Collin, speaking from his operations room at Croydon police station. "There is still the possibility that they aren't the same hand of the same suspect, although I must say that at this stage it's looking like they are."

Collin leads a team of 16 officers. They are running a number of investigations, but it is the cat killer case alone that has been the "continuous thread of the past 10 months" for Collin. "I am answering the phone about it seven days a week, pretty much," he says. "Unless I'm asleep, and even then I sometimes answer the phone, because the phone's never off and it does ring all hours."

While the police currently have a list of "persons of interest", there is no identified suspect. So far no individual person has appeared at multiple crime scenes, or been caught on CCTV more than once near to where a cat's body has been found. In the absence of that, it is hard for the police to narrow down their investigation to any one individual.

But they do have a working "profile" of who this person might be. They suspect a man, not a woman, and someone who has committed other crimes in the past – whether or not they have ever been apprehended before. They also think he might not be solely interested in cats; Snarl has recovered bodies of foxes and other animals that bear similar mutilations.

At first the cases were confined to London, but then Snarl started picking up reports of unnervingly similar incidents in other towns

Collin stresses that he has no evidence to suggest the killer might be about to start targeting people, or carrying out more violent crimes, but he has considered the worst. "Escalation is obviously a concern."

This, and growing public outrage over the mutilations, has tightened Collin's resolve to find the perpetrator. "'Enjoy' is not the right word," he says, "but I'll be very pleased to bring this series to a conclusion and get hold of the right person."

Cases where people have repeatedly targeted and killed domesticated animals are extremely rare. But at the same time that Snarl was first alerted to the UK incidents in 2015, a man in San Jose, California, was caught on CCTV luring an orange tabby cat – a local pet called GoGo – towards him in the street. The man, 24-year-old Robert Farmer, grabbed GoGo and ran off. A few weeks later he was arrested by San Jose police officers and quickly linked to other cat disappearances and killings in the same area.

One of the cats found dead was Thumper, owned by Myriam Martinez. Thumper had gone missing twice. When she returned after the first disappearance, Martinez says her pet did not look right, as though she were in pain. "She looked really bad," remembers Martinez. "We were going to take her to the vet." But Thumper went missing again before the vet could examine her. She was later found dead, lying in a dumpster.

In February 2016, Farmer pleaded not guilty to 10 felony counts of animal cruelty. But as the court case wore on and new evidence was admitted, the charges began to stack up. The police used DNA evidence to show that Farmer had had contact with several cats identified as victims. Officers found four bodies and believe he killed up to 16 cats during his spree.

Eventually, in October 2016, Farmer pleaded guilty to 21 counts of animal cruelty. He is due to be sentenced in early 2017 and faces more than 16 years in prison.

It will be difficult to prove how – or even if – a suspect has killed the cats

For Martinez, a cat owner who got wrapped up in the case, this change of plea was shattering. "It felt really bad, you know?" she says emotionally. "Part of me was very relieved, but part of me hated him so much."

She says she will never own another cat.

The case had come to the attention of Leonor Delgado of the Humane Society in Palo Alto, California. As Delgado notes, repeated targeting of domesticated animals is also "highly unusual" in the US.

"We're all fervently hoping that he will get the maximum sentence," she says. Martinez and other locals are part of a Facebook group, Justice for our CATZ, that is encouraging supporters to write to the judge.

Farmer faces many years behind bars, but his is a separate case in a different country. So what penalties might confront the UK's cat killer if they were ever caught and found guilty?

"Whatever it will be will not be anywhere near enough what it deserves, really," says Mike Butcher.

Collin hopes that the case will one day go before a Crown Court – where serious criminal offences are handled – and that a whole series of mutilations would be included in the case against him.

However, Jenkins and Rising point out that it will be difficult to prove how – or even if – a suspect has killed the cats, so animal cruelty charges would be difficult to bring. Even if such cruelty charges were brought, they only carry a maximum penalty of 12 months in prison.

Two adult, fully-grown men just suddenly burst into tears

However, it is possible a suspect could be charged with criminal damage, since the cats are technically the possessions of their owners. That could carry a much longer sentence.

"Personally, I'd be quite happy if he got 10-15 years," says Rising. "And [was] monitored thereafter."

Jenkins and Rising have not only collected bodies and fielded tip-offs during the investigation over the last year. They have also been in direct contact with many pet owners whose cats have apparently been targeted, so they have seen the impact of the mutilations on families.

"We had one case," says Jenkins. "Two macho guys, father and son. Son was about 30. And their cat had been killed. We collected it the next day about 10 o'clock in the morning. They both broke down in tears in the street."

"That was a member of their family," interjects Rising. "You know?"

Jenkins nods. "That's what struck it home for me, the horror of this," he continues. "Two adult, fully-grown men just suddenly burst into tears."

The killer is still out there. Snarl, the RSPCA and the police continue to receive regular reports of mutilations.

Det Sgt Collin has fielded enquiries from interested criminologists beyond Britain

Some people remain sceptical when they hear the stories. Surely it is just other animals attacking the cats, they say. Why would anyone be mutilating pets on such a scale? But the post-mortem results speak for themselves.

The current advice to pet owners is to make sure that cats are taken in at night. Anyone who finds a cat's body, or who sees someone trying to coax a cat, should tell Snarl or the police.

Jenkins and Rising have landed at the heart of a case that might even impact how serial animal killings are investigated and prosecuted in the UK. And Det Sgt Collin has fielded enquiries from interested criminologists beyond Britain, in continental Europe. There is no doubt that the case is a bizarre one, but it is a serious one, too.

A man, or perhaps a woman, has stalked these Croydon streets – and streets in other parts of Britain – looking for their next victim. You would not think it, to look around at these hushed cul-de-sacs and avenues. But in a house somewhere along one of those roads, it is probable the perpetrator is quietly planning another deadly walk in the night, perhaps with a knife hidden in a coat or bag.

But the cats of London, or any other town, cannot be told to run.

Anyone with information is encouraged to call the Metropolitan Police on 0208 649 0216 or the RSPCA on 0300 123 4999. Snarl can also be contacted on 07961 030064 or 07957 830490. If the mutilated body of a cat is found, readers are requested to first call Snarl.

Join over six million BBC Earth fans by liking us on Facebook, or follow us on Twitter and Instagram.

If you liked this story, sign up for the weekly features newsletter called "If You Only Read 6 Things This Week". A handpicked selection of stories from BBC Future, Earth, Culture, Capital, Travel and Autos, delivered to your inbox every Friday.