The woman has been identified but has yet to be arrested after she admitted what she did to another person, a city prosecutor told AFP on Monday.
The witness was walking a dog when the 63-year-old woman struck up a conversation and “talked about her aversion to dogs, said she knew about poison because she was a chemist, and that she killed the dogs,” prosecutor Oscar Montes de Oca said.
The suspect lives in a neighborhood near the trendy Condesa district, where Mexico Park, the scene of the crimes, is located, de Oca said.
But she has not been detained yet because she was not caught red-handed placing poison-laced meat at the park, a location beloved by dog owners in the mega-capital of 20 million people.
De Oca said investigators are reviewing 278 recordings from street surveillance cameras to find more evidence in the case, which has outraged animal rights groups.
Dog owners organized a protest at Mexico Park on Sunday.
“I really hope that they are doing something about this. I have my doubts about how they have carried out this investigation,” Caroline Owen, a British woman whose dog Daisy is among the victims, told AFP.
While prosecutors say they have received complaints about 11 deaths, the Citizen Front for Animal Rights says 18 dogs have been killed.
Autopsies found that the dogs were poisoned with zinc fluoride, which is usually used in rat poison, the prosecutor said.
A veterinarian told AFP earlier this month that the animals experienced the same symptoms: 20 minutes after a stroll in the park or nearby streets, they began to vomit, suffered convulsions and stopped breathing.
The sentence of killing an animal is two to four yeas in prison in Mexico City.