A two-year-old girl died after eating a poison-laced cookie reportedly thrown into her garden to silence the family's noisy dog.
Heidy Valeria, two, stopped breathing twice while medics battled to save her in Mexico's Jalisco state before her death on February 3.
She was taken to hospital two days earlier after eating part of the poisoned cookie outside her home.
Local official Epigmenio Carrillo said the cause of little Heidy's poisoning is yet to be confirmed.
But the dog and a six-year-old girl, who was reportedly playing with the youngster, were both said to have suffered the same issues, according to local media.
A local resident said: "The truth is, the news has shocked the whole town. In fact, social media erupted when it became known what had happened."
Tototlan residents have been expressing their shock online, and accused a disgruntled neighbour of mistakenly poisoning the youngster while trying to target the family's dog - which barked a lot.
Officials said an announcement would be made as soon the facts become available.
Carrillo said another local six-year-old child was hospitalised due to poisoning from an unidentified substance.
It's still unclear whether this child's condition is related to the deceased toddler.
Piling on pressure for local authorities to swiftly arrest any guilty parties, Mexican actor Arturo Islas Allende said: "We lost Valeria, we lost a two-year-old girl, because of some unscrupulous characters who threw poisoned cookies in Tototlan, Jalisco.
"Those guys should be in jail."
The victim's family have been offered support from local law enforcement as the investigation unfolds, reports say.
