A section of Parque Mexico where dozens of dogs normally play with their owners has been cordoned off while police guard the area and investigate the recent deaths.
A sign was placed at the park with a dire warning: “Red alert. Sudden poisoning of dogs in Parque Mexico. Watch out for your children.”
“We recommend shortening walks until the investigation of the dog poisonings is concluded,” reads another warning.
A veterinary hospital said 11 dogs were apparently poisoned, while the animal rights group Frecda reported 18 deaths in recent weeks.
Built in the 1920s with Art Deco designs, the park is in the capital’s trendy Condesa district, which is known for its nightlife and population of expats, artists and local politicians.
But the park is also known as a playground for pet owners, who throw sticks for their dogs, let them loose in a pond and lavish their animals with baths in mobile pet grooming trucks.
Local media have dubbed the killer, or killers, the “Mataperros de la Condesa”, or “The Dog-Killer of Condesa”.
The Mexico City prosecutor’s office launched an investigation after heartbroken pet owners filed complaints and the deaths caused a furious response on social media.
“We are guarding the area, investigating, asking questions to certain people,” a police officer said on condition of anonymity.
The prosecutor’s office ordered autopsies on the dogs and toxicology exams.
Cleaning crews, meanwhile, used pressure water hoses to clean the park from “any residual poison,” one of the workers said.
Berenice Nimodio, a veterinarian at the Animalia Hospital where many of the victims were taken, said the dogs probably ate food laced with a substance known as fluoride zinc.
All of the animal victims experience the same symptoms: 20 minutes after a stroll in the park or nearby streets, they began to vomit, suffered convulsions and stopped breathing, Ms Nimodio said.
Guillermo Islas, an artist who has sold his work for 30 years at the park, suggested one possible theory.
“There are sick people who do that [place poisoned food in the park], but there are also many dog owners who are hated because they walk to show off and don’t pick up the faeces,” he said.
In a country where tens of thousands of people have died in a drug war in which most crimes against humans go unsolved, the measures taken by the authorities to investigate the dog deaths has raised some eyebrows on social media.
“All life deserves attention,” said Vicente Solano, a construction company worker, admitting that he was “worried and watchful” as he walked his golden retriever at the park.
“All crimes must be investigated, those against dogs and humans. Those are lives that must be respected,” Mr Solano said.
Frecda urged the authorities to review the footage of surveillance cameras around the park and find those responsible for a crime punishable by up to six years in prison.
Caroline Owen, a British woman, said her rescue dog Daisy died at a hospital after she suffered convulsions.
Her dog’s body was among two that were frozen and are being analyzed at the National Autonomous University of Mexico.
For Ms Owen, a group, not an individual, is behind the deaths.
“We won’t give up. For all of us, these dogs were like our children,” she said. “Are they waiting until a child dies [of poisoning] to do something?”