The call came in Friday afternoon: Three children at a tiny, ground-floor day care in the Bronx could not wake up from nap time.

Emergency medical workers arrived at the six-story brick building around 2:45 p.m. to find a 1-year-old unconscious, along with a 2-year-old boy and an 8-month-old girl. The responders at once suspected drugs.

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

Nicholas Dominici, who would have turned 2 in November, was pronounced dead at Montefiore Medical Center on Friday. By early Saturday, the other three children were in critical or stable condition, and the police were questioning a person after discovering equipment typically used by drug dealers on the premises.

Nicholas’s death brought together two crises that afflict New York and the nation at large: working parents’ desperate hunt for affordable, dependable child care and the scourge of opioids such as fentanyl, which contributed to about 75,000 overdose deaths in the United States last year. The Bronx has been hit particularly hard by the drug, which can kill in minute quantities.

“This crisis is real, and it is a real wake‑up call for individuals who have opioids or fentanyl in their homes,” Mayor Eric Adams said at a briefing just after midnight.

On Saturday, at least one person was in police custody and being questioned, according to the police. After an autopsy on Saturday, the New York City medical examiner’s office said further examination was needed to determine Nicholas’s cause of death. The police did not name the person or people whom they had in custody on Saturday.

Joseph E. Kenny, the Police Department’s chief of detectives, said at the news briefing 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.

There were 2,668 fatal overdoses in the city in 2021, reaching “unprecedented levels,” according to data released by the city this year. The increase was driven by fentanyl, a synthetic opioid that was involved in 80 percent of overdose deaths that year, and residents of the Bronx had the highest rate of deaths, the city found. The day care, Divino Niño, is in the 52nd Precinct in the northern portion of the Bronx, which is among the areas hardest hit by fatal overdoses.

It was not clear how the children could have come in contact with any drugs. Nearly all cases of children being exposed to opioids in the United States involved the children ingesting the drug, a 2019 study in the Journal of Pediatrics found.

The study looked at more than 80,000 records of children who had been exposed to opioid-containing drugs from 2010 to 2014, and found that roughly 99 percent of the exposures involved children ingesting the drug.

The other routes of exposure included inhalation or contact with children’s eyes, but the data in the study was largely self-reported, making it difficult to determine if those types of exposure would have been enough to sicken children.

At the midnight news conference, Dr. Ashwin Vasan, commissioner of the city’s Department of Health and Mental Hygiene, said that “a small child — not someone we would think would be at risk of interacting with opioids — has come into contact with a powerful substance.”

“What it tells us is that the overdose crisis affects all of us, which is why it’s an all-hands-on-deck public health moment,” he added. “Our hearts go out to the family for their loss.”

Nicholas’s father said in an interview with WCBS-TV that the boy, the youngest of five, had been at the day care for a single week and was just getting acclimated. The day care had passed its inspections and was recommended, and the family had endured a wait list, Nicholas’s father, Otoniel Feliz, said in Spanish.

“The hardest thing for me is to come home and open that door and not see Nicholas saying, ‘Dad, Dad,’” Mr. Feliz said.

The boy’s mother, Zoila Dominici, said her son was “so intelligent. He would repeat everything you would say to him. He had so much love.”

The day care at 2707 Morris Avenue where Nicholas died, half a mile from his home, was registered in May and had capacity for eight children between 6 weeks and 12 years old, according to public records.

Calls to a number listed for the day care were not immediately returned on Saturday. A woman who answered a phone for Grei F. Mendez De Ventura, a person listed as a contact for the location, said she did not wish to be interviewed.

Officials said the day care had been licensed by the state’s Office of Children and Family Services after passing two inspections. A “surprise” inspection last week by the city’s health department on behalf of the state agency found no violations, Dr. Vasan said. Unannounced inspections of licensed child care providers are standard procedure and do not necessarily indicate that a problem is suspected.

A spokesman with the state agency, Solomon Syed, said he could not comment on an investigation.

Tiny day cares in ground floors of apartment buildings are a common sight in working-class neighborhoods in New York, where parents struggle to earn enough to pay for care and providers themselves try to eke out a livable income.

Anna Ortiz-Irving, 73, who lives next door to Divino Niño, said she was friendly with the mother and daughter who she said own it, and that they had worked hard for months to spruce it up, laying down new floors and putting up walls.

The windows of the day care are covered with blue metal grates, a Minnie Mouse sticker adorning one window covering. Behind curtains decorated with coffee mugs, scented candles sit on the window sills next to a collection of books.

“It is not a basement,” Ms. Ortiz-Irving said. “It is a walk-in apartment. You could walk by and look right in and see how nice everything was. She always had the window open. They had gates, but you could look right in and see how beautiful it was inside.”

The building on Morris Avenue is just off the busy shopping thoroughfare of Kingsbridge Road. On Saturday, children were everywhere: riding in strollers on the avenue and skipping in colorful leggings down sidewalks, their shouts ringing out through an otherwise quiet neighborhood.

On Friday evening, neighbors had 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.

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.”

As the rapid spread of fentanyl has propelled a grim death toll in New York City and elsewhere, young children have not been spared.

Opioids were the leading cause of poisoning deaths in children five years old and younger from 2005 to 2018, a study in the journal Pediatrics found.

The study, published in March, looked at 731 poisoning-related deaths across 40 states. The authors found that opioids, a class of synthetic drugs that includes prescribed pain relievers but also illegal narcotics such as heroin and fentanyl, contributed to nearly half, or 47 percent, of those deaths.

In just a few months in 2021, fentanyl and other opioids were linked to the deaths of an 11-month-old girl in South Carolina, a 10-month-old in Pennsylvania, a 2-year-old boy in Indiana and a 15-month-old boy in California.

In New York, a 22-month-old boy died in June 2021 after his father fed him a bottle of fentanyl-contaminated formula in the apartment where they were living at a homeless shelter on Lower East Side of Manhattan, according to a criminal complaint charging the father with manslaughter. The boy’s body contained enough of the drug to kill an adult, officials said.

In November of that year, the 10-month-old granddaughter of the novelist Paul Auster died of an overdose of fentanyl and heroin. The girl’s father, Daniel Auster, was charged in her death; the case was pending when he also died after a drug overdose.

And in February, the death of a 16-month-old boy on Staten Island was ruled a homicide after an overdose on a combination of fentanyl, acetyl fentanyl and cocaine.

In the Bronx on Saturday, it seemed another child would be added to the toll. The only hint of a new casualty in the opioid epidemic was tied to the blue gate leading to the main entrance of the building: a small piece of yellow caution tape.

Benjamin Mueller and Erin Nolan contributed reporting.