Two days after Christmas, Darwin Santana-Gonzalez, a curly-haired 1-year-old, was toddling around a Bronx apartment where, the police said, a potent mixture of heroin and fentanyl was being prepared, stamped and packed for sale.

The powerful opioids had been placed in packages, the authorities said, along with a related drug, acetylfentanyl, creating the sort of deadly cocktail that has led to a surge of overdose deaths in the Bronx and beyond. Somehow, some of the mix also ended up in Darwin.

By 10 a.m. that morning, the police said, the little boy had stopped breathing and was dead.

Darwin’s parents, who had flagged down the police as they tried to rush him to the hospital, were charged in January with multiple counts of drug possession, but his father fled the country. On Wednesday, the police announced that his mother, Daira Santana-Gonzalez, who was in custody, had been charged with murder in her son’s death.

Darwin’s death is a reminder of the pitiless power of fentanyl, a drug that can kill children even through accidental ingestion of very small quantities. It is not known how Darwin ingested the drugs.