A day care center operator and her neighbor were charged with murder in the death of a 1-year-old boy who was exposed to opioids at the center, the police said.

Grei Mendez, 36, who ran Divino Niño in the Bronx, and Carlisto Acevedo Brito, a 41-year-old man who lived at the address listed for the day care center, were both arrested Saturday evening on a count of murder showing “depraved indifference,” four counts of endangering the welfare of a charge, 16 counts of assault, and criminal possession of drugs.

Emergency medical workers arrived at the six-story brick building in the north Bronx around 2:45 p.m. on Friday to find Nicholas Feliz Dominici, who would have turned 2 in November, unconscious. They also found a 2-year-old boy and an 8-month-old girl, both “unconscious and unresponsive,” the police said.

All three children showed symptoms of opioid exposure, the police said.

They gave the young children the overdose-reversal medication Narcan and took them away. Another 2-year-old-boy, who had left the small ground-floor day care center shortly after noon, was taken to a hospital after his mother noticed an unusual lethargy had replaced a toddler’s normal energy.

Nicholas was pronounced dead at Montefiore Medical Center on Friday. By early Saturday, the other three children were in critical or stable condition.

Joseph E. Kenny, the Police Department’s chief of detectives, said at a news briefing on Friday that suspicions about opioid exposure were prompted by the children’s symptoms and by the discovery of a so-called kilo press — commonly used by drug dealers when packaging large quantities of drugs — at the day care during a search.

The opioid was not identified, but during Friday’s briefing, Mayor Eric Adams and Ashwin Vasan, the commissioner of the department of health and mental hygiene, described the dangers of fentanyl and the drug’s pervasiveness.

“This crisis is real, and it is a real wake‑up call for individuals who have opioids or fentanyl in their homes,” Mayor Adams said. “The mere contact is deadly for an adult and it’s extremely deadly for a child.”

The boy’s mother, Zoila Dominici, said on Sunday that she arrived at the day care at 3 p.m. on Friday to pick her son up when she saw the police surrounding the building and Ms. Grei inside crying.

Then she got a call from a coordinator of the program telling her Nicholas, who had only been at the day care center five days, was in the hospital.

Nicholas, the youngest of five children, was a smart boy who spoke and learned to count at an early age and was walking by the time he was 10 months old, said Ms. Dominici, a 34-year-old caregiver for older people.

“The other children adored him. He was the little prince of the house,” she said. “He shouldn’t have died like that.”