A 1-year-old boy died on Friday and three other children were hospitalized after all four were apparently exposed to an opioid at a state-licensed Bronx day care where the police later found a packaging device typically used by drug dealers, officials said.

The New York City medical examiner’s office said it expected to reach a determination about the child’s cause of death later on Saturday, but Mayor Eric Adams said at a news briefing streamed online overnight that the four children “appear to have come into contact with an opioid.”

Joseph E. Kenny, the Police Department’s chief of detectives, said at the briefing that the episode was the subject of an “active criminal investigation.” He added that suspicions about opioid exposure were prompted by the children’s symptoms and by the discovery of a so-called kilo press at the day care site during the execution of a search warrant there.

“This is an item that is commonly used by drug dealers when packaging large quantities of drugs,” Chief Kenny said.

Fentanyl, a powerful synthetic narcotic that is increasingly turning up in the illicit drug supply in United States, was not identified as the potential culprit. But in his remarks at the briefing, the city’s health commissioner cited it as the possible cause.

The day care where the episode occurred opened in January with a license from the state’s Office of Children and Family Services after passing two inspections, said the commissioner, Dr. Ashwin Vasan. A “surprise” inspection last week by the city’s health department on behalf of the state agency found no violations, he said.

Chief Kenny gave the following account of the episode at the day care:

Emergency medical workers responded to a 911 call requesting help for three unconscious children at 2707 Morris Avenue in the Bronx around 2:45 p.m.

Finding the children — the 1-year-old boy, a 2-year-old boy and an 8-month-old girl — unresponsive and showing symptoms of opioid exposure, the emergency workers administered the opioid-reversal medication Narcan and took them to Montefiore Medical Center.

The girl and the 2-year-old boy regained consciousness; as of early Saturday he was in critical condition, and she was in stable condition. The 1-year-old boy, whom the police identified as Nicholas Dominici, was pronounced dead at the hospital. He would have turned 2 in November.

The police learned that a fourth child, another 2-year-old boy, had left the day care shortly after noon. Once he was home, his mother noticed that he was lethargic and unresponsive.

The boy was taken to BronxCare Health System hospital, where he, too, was in stable condition after being revived with Narcan.

(The police had initially reported that all four children were at the day care when emergency workers first responded and that the 1-year-old had been pronounced dead there.)

After the children were removed from the day care, Fire Department units tested the premises for environmental hazards and found no evidence of carbon monoxide, officials said.

Calls and text messages to the contact number for the licensed day care at the Morris Avenue address were not immediately returned.

On Friday evening, neighbors on the largely residential tree-lined block peered out of open apartment windows as the flashing light of an emergency services vehicle reflected off glass panes, and investigators stood near a sealed-off, 30-foot radius around the day care.

Anna Ortiz-Irving, 73, who lives next door to the day care, said she was friendly with the mother and daughter who own it. The business opened only a few months ago, she said, but they had worked hard for several months before that to update the ground-floor apartment and ensure it was up to city and state code. She said the inside of the day care was visible from the street and described it as “beautiful.”

Ms. Ortiz-Irving said that a neighbor told her that sometime after 2 p.m. one of the women who operated the day care ran outside and screamed for help because she was unable to wake the children up from a nap.

“Somebody called 911, but she was panic-stricken,” she said.

“I don’t know what happened,” she added. “All I can tell you is her and her mother are decent people.”

With the United States in the continuing grip of an opioid crisis, the rapid spread of fentanyl has helped fuel the death toll in New York City and elsewhere. Young children have not been spared.