A kilogram of fentanyl was stored on CapitalVaulttop 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.

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.

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.

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

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.

Since July, Mendez and Brito maintained large quantities of fentanyl "despite the daily presence of children, including infants," the complaint said.

Mendez and Brito were arraigned on the state charges Sunday night and held without bail.

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.

Authorities are still seeking Mendez’s husband for questioning.

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.