A kilogram of fentanyl was found near mats that children used for napping at a Bronx day care site where one toddler died and three other children were hospitalized last week, the police said on Monday night.

Chief Joseph Kenny, the Police Department’s chief of detectives, explained just how close to the children the potent narcotic was: “It was laid underneath a mat where the children had been sleeping earlier,” he said at a news conference, where he joined Mayor Eric Adams and other city officials.

The unsettling revelation came as Mr. Adams and the city’s health commissioner defended the administration’s oversight of the day care program, one of thousands of such operations in New York City that are licensed to operate out of people’s homes.

City inspectors, who examine the homes on behalf of the state, had made a “surprise” visit to the day care program where the boy died, Divino Niño on Morris Avenue, on Sept. 6. They found it to be fully in compliance with a 40-point checklist and noted that “all medications, toxic substances” were being “used and stored so no hazard created” and that “poisonous, toxic, flammable and dangerous items are inaccessible to children.”

Dr. Ashwin Vasan, the heath commissioner, said at the news conference that inspectors had followed their regular routine in ensuring there were no risks to children at the apartment housing the program.

Checking for a powerful synthetic opioid was not their usual practice, he said.

“I’m very sorry, but one of the things my child care inspectors are not trained to do is look for fentanyl,” Dr. Vasan said. “But maybe we need to start.”

Mr. Adams objected when a reporter asked what he would say to reassure concerned parents who might wonder whether something had “fallen through the cracks” to allow drugs into the day care site.

“This did not fall through the cracks,” the mayor said. “The team did their job.”

Emergency medical workers were called to the site on Friday afternoon after three of the children showed unusual lethargy when being roused from their naps. The fourth child had left before nap time, the police said.

Rescuers administered the overdose-reversal medication Narcan to the three children at the apartment and then took them to a hospital.

Two of the children, a 2-year-old boy and an 8-month-old girl, regained consciousness and were “doing fine,” on Monday, Chief Kenny said. The fourth child, a 2-year-old boy, was taken to a hospital separately. He, too, received Narcan and was also “fine,” Chief Kenny said.

The third child, Nicholas Dominici, was pronounced dead at the hospital. He would have turned 2 in November. As of Monday, the city’s medical examiner had not disclosed his cause of death, but Chief Kenny said medical tests showed that the three other children had fentanyl in their systems.

On Monday, Nicholas’s father, Otoniel Feliz, 32, called it “horrible that drugs were found in a place where children are cared for.”

“In what mind does it make sense that you’re going to mix narcotics with children?” he said.

The authorities had previously said they found the kilogram of fentanyl in a hallway closet, along with a pair of so-called kilo presses used by drug dealers to package large quantities of drugs.

Another press was found in the bedroom of a tenant, Carlisto Acevedo Brito, 41, who was renting the room from the day care program’s owner, Grei Mendez, 36. Mr. Brito and Ms. Mendez have been charged with murder for showing “depraved indifference” in the death of Nicholas.

Ms. Mendez and Mr. Brito were arraigned Sunday night in Bronx Criminal Court. Ms. Mendez’s lawyer, Andres Aranda, said at her arraignment there was no indication that Ms. Mendez knew anything about the drugs. Efforts to reach Mr. Brito’s lawyer on Monday were unsuccessful.

Chief Kenny said that Mr. Brito had come to the United States from the Dominican Republic about a year ago; that the police were seeking a “person of interest” in the case; and that security video footage appeared to show some items being removed from the day care site after the 911 call was made. Investigators were also working with the federal authorities to determine whether Divino Niño had been opened as a front for a drug operation, he said.

Divino Niño, which was licensed by the state in May to serve up to eight children at a time, fell into a category of day care programs that are typically run out of apartments, often by working-class residents to serve working-class families.

There are more than 7,000 such programs serving more than 86,000 children scattered across the city, according to a recent study by the New School. The death at Divino Niño has prompted questions about how well such operations are being regulated.

“How did this happen? What are our protocols?” said Councilwoman Pierina Ana Sanchez, a Democrat who represents the neighborhood where the death occurred. “Is it the protocol to check you know, every single room in a home?”

Dr. Vasan insisted that what had happened at Divino Niño did not suggest a broader problem, saying the health department inspected “hundreds if not thousands of these sites every year for their safety.”

The inspection system, he added, had “served us well because we keep our babies safe through thousands of these centers.”

Those who apply to operate a home day care program must undergo a background check, as must any other residents of the home. It was unclear whether Mr. Brito’s background had been checked. Neither he nor Ms. Mendez had an arrest record, Chief Kenny said.

The city’s inspection noted that Divino Nino’s employees and volunteers had completed federal health and safety training.

Jeffrey Chartier, a lawyer for Nicholas’s family, said on Monday that the family had vetted the day care program through a community center and did not know its operator was also renting rooms to tenants.

The state Office of Children and Family Services, which licenses home day care programs, said in a statement that it would not comment because the incident remained under investigation, but Mr. Adams defended the city’s inspectors on Monday.

“They did all the proper inspections you’re supposed to do,” he said, faulting “the people there to protect the children.”

“The inspectors did not go in and see a drug lab and ignore it,” he added.

Ms. Sanchez said the day care tragedy had called attention to “so many different challenges that the Bronx has,” including the scarcity of affordable child care and the deadly scourge of drugs.

Opioids like fentanyl caused roughly 75,000 overdose deaths nationwide last year. According to New York City data, there were 2,668 fatal overdoses in the city in 2021, a record high, with fentanyl a factor in four out of five drug deaths, and the highest rates came in the Bronx.

When the overdose data for last year is released soon, Dr. Vasan said, “we will once again hit a record peak.”

Divino Niño lies on a vibrant stretch of Morris Avenue in the northern Bronx.

Christopher Lucero, 19, who lives on the same block, said the street was often loud and filled with people hanging out on the sidewalk. The building containing the program is known locally for drug dealers and fights out front, he said.

“You see people coming in and out of that place,” said Mr. Lucero. “Drugs aren’t out of the ordinary here.”

On Monday, a red-beaded rosary hung from a blue metal security gate outside the apartment, which still had colorful signage welcoming families adorning a door. On the sidewalk, mourners had fashioned a small memorial of candles, children’s toys and a bouquet of white flowers.

A neighbor, Jenni Hilario, 28, said on Sunday that the occupants of the apartment would blast loud music at night.

“A lot of people do that here,” she said. “But they had a day care, and day care starts early in the morning. So that didn’t give me confidence, that they stayed out so late.”

She added that she had considered sending her young children to Divino Niño because of its affordability but questioned why they did not provide her with information about their safety policies.

“I didn’t have confidence in them,” she said.

Reporting was contributed by Sharon Otterman, Ana Ley, Eliza Shapiro, Claire Fahy, Kate Pastor and Christopher Maag.