Police on Thursday returned to the New York City day care where a 1-year-old boy died following exposure to fentanyl to search for potentially hidden drugs, sources told ABC News.

Nicholas Dominici, 1, died on Friday and 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.

Authorities' searches on Wednesday night and Thursday followed a tip they received about a trap door in the floor of the Bronx day care, law enforcement sources familiar with the case told ABC News.

Investigators had already found a kilo of fentanyl stored on kids' play mats, along with a device to press drugs into bricks for sale, according to court records.

Grei Mendez, the operator of the day care, and her tenant, Carlisto Acevedo Brito, are in federal custody on charges of narcotics possession with intent to distribute resulting in death and conspiracy to distribute narcotics resulting in death. They've been held without bail.

Mendez and Brito were initially arrested on state charges including murder. The state case has been continued to Oct. 5.

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.

A search is ongoing for Mendez’s husband, who, according to court records, was seen on video fleeing the day care out of a back alley carrying two trash bags.

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.