LONDON — The bodies have been dumped on doorsteps with chilling regularity. There was Oscar, an 8-year-old tabby, his head and tail cut off; Oreo, a Siamese kitten left at the bottom of a garden, decapitated and with her collar lying across her body; and Paddy, who had been hacked in half. The missing heads have not been found; they might have been kept as trophies.

The gruesome killings and mutilations of scores of cats in London over the past six months, starting in the South London neighborhood of Croydon, are stoking fears of a dangerous sadist in this city of animal lovers. Several newspapers and broadcasters have taken to calling the killer the Croydon Cat Ripper.

“Because animals cannot report incidents of abuse against them and can do little to fight back, they are the perfect practice victims for violent people,” said Ms Elisa Allen, associate director of the British chapter of People for the Ethical Treatment of Animals, or PETA. “The pain and fear that any of these cats experienced is unimaginable.”

PETA is offering a reward of just over US$7,000 for information leading to an arrest.

The pattern of attacks first came to light last fall when South Norwood Animal Rescue and Liberty, a local organization, began to document the cat deaths. Mr Tony Jenkins, who runs the group with his girlfriend, Ms Boudicca Rising, said they had documented 46 cats that they believed had been killed, possibly by the same person or people.

After each killing, the couple, who have been working with the police, have gone to the scene to photograph and retrieve the body.

“The cat killer is clever, operates in darkness in mostly residential neighborhoods away from surveillance video cameras,” said Jenkins, adding, “The aim is obviously to inflict maximum distress.”

When reports of the killings first emerged, the Royal Society for the Prevention of Cruelty to Animals, Britain’s best-known animal welfare charity, treated them as car accidents followed by mutilations by foxes, which are ubiquitous in London.

But as the bodies piled up, Mr Mike Butcher, an investigator at the charity, asked a specialist at the Royal Veterinary College to perform autopsies on 20 of the cats. In 44 years of investigating animal cruelty, he said, he had never seen such systematic brutality.

Scotland Yard has confirmed that at least 20 cats have been killed, and that at least 19 of the killings appeared to be “linked.” Mr Alex Stoll, the veterinary forensic pathologist who conducted the post-mortems, said the cats had died from “blunt force trauma” before being mutilated by a “sharp implement, probably wielded by a human being.” (He emphasized that he did not rule out the possibility that the cats had first been run over by a car.)

Swabbing a cat for DNA traces, Mr Stoll added, would be unlikely to help catch the culprit because cats are petted by so many people.

No arrests have been made.

“At this stage, the exact nature of any offense committed is not clear, and the ongoing investigation is committed to seeking any evidence or information that will assist this,” the Scotland Yard said in a statement.

While there is no evidence to suggest that humans are at direct risk, some notorious serial killers * for example, the so-called Boston Strangler, Albert H. DeSalvo, who confessed to killing 13 women in the early 1960s; and Jeffrey L. Dahmer, whose acts of murder and necrophilia came to light in 1991 * began with animal cruelty.

The string of cat killings in London has left many of the pets’ owners traumatized.

Ms Naomi Chauhan, 27, an events manager from Brackley, a town in south Northamptonshire, was so enamored of her 1-year-old white and ginger cat Ivy that she splurged on a pet sofa, a drinking fountain and a special electronic cat flap that recognizes a pet’s ID chip in an effort to keep unwanted visitors away.

“My cats are my babies,” she said.

When Ms Chauhan discovered that Ivy was missing on a Thursday evening last month, she searched everywhere. The next morning, she combed the neighborhood in her car and spotted a cat lying near a road, 500 yards from her home. When she got closer, she was overcome by horror: Ivy had no head.

“It was very clean and clinical,” she said. “This was no accident. There was no blood. A fox wouldn’t have done it that way.”

Ms Chauhan says she has fallen into depression and has not been able to go to work since the discovery.

“How could anyone do this?” she asked.

In Britain, animal cruelty can carry a fine of up to about $29,000 and a sentence of up to six months in prison. After the cat killings, animal-rights groups have been calling for tougher penalties and for the creation of an offense of “animal murder.”

In the United States, the FBI recently began counting animal cruelty in its annual statistics on felonies and other crimes.

In Croydon, Jenkins, from the South Norwood animal welfare group, has appealed for information from the public. The killings have spawned no shortage of conspiracy theories. Tips have poured in, with some of the more outlandish ones blaming satanic worshippers or the Islamic State. While the killings remain unsolved, cat flaps are being nailed closed, pets shut indoors and difficult news kept from children.

“My daughter kept asking when Barney was coming home,” said Ms Danielle Bowden, a 42-year-old from southeast London, whose cat was beheaded. She told her three young children that Barney had been struck by a car. THE NEW YORK TIMES