More than 60 dogs - many of them pets - have been poisoned to death in the last few weeks in the Mexican city of Hermosillo, leading to demands for an inquiry from animal rights activists.
Many of the pets have been killed in their own homes by meat, laced with rat poison or insecticide, that has been tossed over walls and gates.
Several stray animals are also thought to have been killed.
It is unclear if the police has the resources to investigate the deaths.
Hermosillo, an industrial city of some 800,000 people in northern Mexico, is plagued by violence and drug crime.
The poisonings came to light in mid-March.
According to the Associated Press news agency, the authorities were stunned when 10 dog deaths were reported in a single day - as many as the city usually registers in a month.
"Everyone here believes that it is only one person performing these actions," animal rights activist Carolina de la Torre told AP.
According to the agency, a Los Angeles-based actor, Raul Julia Levy, has offered a $10,000 (£6,700; €9,200) reward for information leading to an arrest.
Local media have nicknamed the person responsible, "mataperros" - "the dog killer".