(NewsNation) — The federal agent overseeing the investigation into fentanyl found inside a Bronx day care says the Drug Enforcement Administration will “stop at nothing” to find the third person they say was involved in a drug trafficking operation.

Federal prosecutors have charged Grei Mendez and her husband’s cousin, Carlecito Brito, days after babies were exposed to fentanyl inside a Bronx day care, killing one of them. Mendez is accused of calling her husband twice and another person once before dialing 911 to help three unresponsive children who didn’t wake up from their nap Friday.

One of the babies, 1-year-old Nicholas Dominici, died, while two other siblings — 8 months and 2 years old — were hospitalized and revived with naloxone, a lifesaving drug that reverses the effects of opioid overdoses. A fourth child was also exposed and treated at a hospital.

Mendez’s husband remains on the run.

“We’re committed to tracking this person, from Broadway in New York City all the way back to Main Street Mexico, or Main Street Dominican Republic, or wherever he resides,” said Frank Tarentino, special agent in charge for the DEA in New York. “We will stop at nothing to hold that person accountable and bring him into custody.”

The criminal complaint alleges surveillance footage shows Mendez’s husband going into the day care empty-handed before first responders and police arrived. Two minutes later, authorities said, the husband was seen running out of a back alley of the day care carrying two shopping bags weighted down with contents.

Authorities said the drug operation at the day care on Morris Avenue was ongoing from July through September. The complaint also said Mendez deleted approximately 21,526 messages from an encrypted messaging app that covered a period between March 30, 2021 and last Friday.   

“Despite the daily presence of children, including infants, the defendants maintained large quantities of fentanyl, including a kilogram of fentanyl stored on top of children’s playmats,” the criminal complaint stated.

After the initial bust, police went back Wednesday night to search the property and found a “trap door” of sorts in the floor that was used to conceal drugs.

“This investigation is not that dissimilar than other investigations that we see play out across the country and here in New York City, where large wholesale distributors of different types of drugs like fentanyl, like methamphetamine, like heroin, like cocaine, are utilizing different types of stash facilities, stash houses and different methods of concealing their drugs,” Tarentino said.

He called the proliferation of fentanyl “the most urgent threat” to the United States. According to the Centers for Disease Control and Prevention, nearly 71,000 drug overdose deaths involved synthetic opioids, accounting for 75% of the total overdoses.

Fentanyl is a highly potent synthetic opioid that has the ability to kill someone with as little as 2 milligrams. The pills are cheap to produce, making it a favored product among Mexican drug cartels that then traffic it across the southern U.S. border.

“This is deliberate calculated treachery by the cartels in Mexico,” Tarentino said.

When law enforcement searched the playroom, bathroom, kitchen and bedroom that made up the premises of the day care, they said they found both large quantities of fentanyl as well as machinery and paraphernalia used to package narcotics.   

The federal charges filed against Mendez and Acevedo Brito carry a maximum sentence of life in prison and a minimum sentence of 20 years in prison.

“As law enforcement officers, we’re committed to upholding the rule of law, and we’re committed to holding people accountable who are moving this poison into the United States,” Tarentino said.

NewsNation affiliate WPIX-TV contributed to this report.