Somebody is systematically poisoning the dogs of Hermosillo, an industrial city in northern Mexico, and not just strays: At least 64 dogs, all with owners, have died of a similar poison since mid-March. More stray animals have probably been killed, but had no one to file a complaint, authorities say.

An organic phosphate compound, possible an insecticide or rat poison, apparently was used in most of the cases, and local media have dubbed the person responsible the "Mataperros," or "The Dog Killer."

Not even dogs kept behind the walls of their owners' homes are safe: Officials say the killer has tossed poison into the gated patios of some homes.

No one knows who the dog killer is, whether the killings involve more than one person or what caused them to turn against dogs.

A male caller to a local radio station in Hermosillo claimed to be, along with accomplices, the killer. But he complained about loose dogs, dog bites and dogs spreading disease and uncleanliness — complaints that don't jibe with attacks on pets inside their owners' homes.

Animal rights activist Carolina de la Torre said she doubts that one person could have poisoned so many dogs. But she noted there appears to be a modus operandi: poison wrapped in a hot dog or meat as bait.

"This is systematic. This can't be the work of one person alone," said De la Torre, who says a total of at least 71 dogs have been killed in the city of about 800,000. She said the killings appear to be concentrated in three neighborhoods on the city's south side.

"It could range from a neighbor who is bothered by noise (from pets), or even thieves who want to get rid of the dog in order to be able to break into the house," said De la Torre. "Those are the two theories we are looking at."

Hermosillo resident Julieta Robles, 23, lost her 5-year-old female German Shepherd, "Box," to the poisoner two weeks ago. The dog had gotten out of her home, but was wearing a collar and tag.

"When she came home that night, she was disoriented," Robles said. We tried to help her, we took her to the vet, but we couldn't save her."

"It was a feeling of a lot of helplessness," Robles added, "not knowing who they are or how to respond to a mass poisoning."

Animal defenders are starting to fight back.

Los Angeles-based actor Raul Julia Levy has offered a $10,000 reward for information leading to the arrest of the culprit or culprits.

"When have you heard of anything like this?" Julia Levy said, "We know there are serial killers of humans, but we've never heard of a serial killer of dogs."

While killing a dog is considered a non-serious crime in Hermosillo, punishable by a fine of about $225, the dog killer has introduced poison into people's homes, a much more serious crime involving trespass and risk for the human inhabitants that could carry a four-year sentence.

The killings started to come to light in mid-March. While an average of about 10 dogs, mainly strays, are found dead in Hermosillo each month, authorities were stunned when 10 dead dogs were found or reported in one day.

Jose Luis Icedo, director of the city's animal health center, said veterinarians were able to identify the poison as a probable pesticide.

"It is very easy for anyone to go to a hardware store and buy rat poison," said Icedo, who said at least 64 dogs have been poisoned and acknowledged the true number could be much higher.

"In the case of strays, there is nobody to file a complaint," said Icedo. While stray killings have occurred in the past — strays probably account for about 25 percent of the city's 140,000 dogs, according to conservative estimates — Icedo said with the killings over the last month, the killer "stepped over the line, to inside the home."

It's not clear how much attention authorities can dedicate to the case. Hermosillo is the capital of Sonora state, which is beset with larger problems of drug gangs and common crime.

"We have enough other things on our plate for someone to come along and start doing this to animals," said state prosecutor Carlos Navarro.