Friday, Nicholas Dominici, who had only been at the nursery for a week, died of a suspected drug overdose.

At the Bronx daycare center, three other children were exposed to the powerful narcotic.

Two people have been charged with drug conspiracy and murder.

It is believed that the children, aged eight months to two years old, inhaled fentanyl at the nursery.

Narcan was administered to three children who had overdosed on opioids.

During a search of the nursery, one kilo of fentanyl was found “underneath a mat where the children slept earlier”, NYPD chief detective Joseph Kenny said.

Three presses allegedly used to package kilos of drugs were also found by investigators. The owner of the Divino Niño nursery in the Bronx, Grei Mendez, 36, and her tenant, Carlisto Acevedo Brito, 41, are facing federal charges of narcotics possession with intent to distribute resulting in death, and conspiracy to distribute narcotics resulting in death.

“We allege that the defendants poisoned four babies and killed one of them while running a drug operation from a daycare center,” Manhattan US Attorney Damien Williams said on Tuesday.

Children should be kept safe at daycare centers, not surrounded by drugs that can kill them instantly.”

According to police, the drugs recovered from the nursery could have killed 500,000 people.

US drug deaths have been blamed on fentanyl, a synthetic painkiller 50 times more powerful than heroin.

After finding the children ill, Ms Mendez called her husband before calling 911, according to surveillance footage and phone records. Several full shopping bags were then removed from the nursery by her husband, officials said.