Kilo of fentanyl stored on top of kids' play mats at day care where 1-year-old died: Prosecutors

A kilogram of fentanyl was stored on top of children’s play mats used for napping at the New York City day care where a 1-year-old boy died from exposure to the drug, according to a new federal criminal complaint.

Three other children, ranging in age from 8 months to 2 years, were hospitalized and treated with Narcan and are now recovering, police said. An analysis of urine from one of the victims confirmed the presence of fentanyl, officials said.

Day care operator Grei Mendez and tenant Carlisto Acevedo Brito are now facing federal charges of narcotics possession with intent to distribute resulting in death and conspiracy to distribute narcotics resulting in death, according to federal prosecutors.

In a case that's "shocked the conScience of the city," the defendants allegedly "poisoned four babies and killed one of them because they were running a drug operation from a day care center" in the Bronx, Damian Williams, U.S. Attorney for Southern District of New York, said at a news conference Tuesday.

"Every New Yorker should be outraged," said Frank Tarentino, special agent in charge of the Drug Enforcement Administration's New York Division.

Mendez called 911 on Friday afternoon reporting that several children were unresponsive at her facility.

Seconds before Mendez called 911, she allegedly called a co-conspirator, Williams said. Minutes later, a co-conspirator arrived at the day care, and he then fled carrying two full shopping bags, Williams said, as the children suffered from fentanyl poisoning.

Mendez and Brito were initially arrested on state charges, including murder, manslaughter and assault, following the "reckless, depraved" death of 1-year-old Nicholas Dominici, prosecutors said.

Since July, Mendez and Brito maintained large quantities of fentanyl "despite the daily presence of children, including infants," the complaint said. They also allegedly had machinery used to press and package drugs, prosecutors said.

Tarentino said fentanyl "is now in everything and everywhere, killing victims instantly and indiscriminately," and he called it "the most urgent threat in our nation."

Mendez and Brito were arraigned on the state charges Sunday night and held without bail. Williams said they are now in federal custody and face a maximum of life in prison if convicted.

Williams said authorities are still looking for a co-conspirator who has not been charged at this time.

"We're not done," Williams said. "Look out for more on that soon."

Mendez’s attorney said she was unaware drugs were being stored in her day care by Brito, her husband's cousin, to whom she was renting a room for $200 a week.

The day care was licensed on May 16 by the state’s Office of Children and Family Services, according to public records. It's listed as having a capacity for eight children from 6 weeks old to 12 years old.

City Health inspectors conducted a surprise inspection of the facility on Sept. 6 and did not find any violations, according to City Health Commissioner Ashwin Vasan.

"I'm very sorry, but one of the things that my child care inspectors are not trained to do is look for fentanyl. But maybe they need to," Vasan said at a news conference Monday evening.

ABC News' Emmanuelle Saliba contributed to this report.