A Oaxaca woman has admitted to killing her four children, all of whom were found dead on Monday after apparently being poisoned.

Three girls aged under 10 and a baby boy were found dead on a bed at their home in Chicapa de Castro, a Zapotec community in the Isthmus of Tehuantepe region. Their father made the shocking discovery when he got home from work Monday afternoon, local authorities said.

Local authorities also said that the children’s mother accepted responsibility for the minors’ death, but didn’t say how she killed them.

Local media reports said the children had been poisoned, probably with rat poison. That hypothesis will have to confirmed by autopsies.

The 27-year-old mother, identified as Arely J., received medical treatment Monday for a stab wound to the neck, the newspaper El Universal reported. It was unclear how she sustained the injury, but at least one report suggested it was self-inflicted.

The Oaxaca Attorney General’s Office FGEO said in a statement that it had launched an investigation into the multi-homicide in Chicapa de Castro, located in the municipality of Juchitán.

“The FGEO is committed to carrying out a meticulous investigation taking all evidence into account,” it said.

Chicapa de Castro authorities declared three days of mourning for the deaths of the four children.

“It’s a tragedy that fills us with sadness,” one resident told El Universal. “… The children were innocent beings, little siblings. We won’t forget this tragedy.”

With reports from El Universal, La Razón and Reporte Indigo