Jenna O'Connor, 32, of Ocean Gate, was charged with endangering the welfare of a child in connection with the incident on July 17.
Detectives responded to Community Medical Center in Toms River for a report of a 2-year-old girl in critical condition.
The child was airlifted to Robert Wood Johnson University Hospital in New Brunswick for further treatment, and on July 21, the child died, prosecutors said.
Prosecutors said tests performed at the hospital revealed methadone in the child's system.
An initial investigation by the Ocean County Prosecutor's Office Major Crime Unit, Special Victims Unit and Police Detective Bureau revealed that on July 17, O'Connor called 911 after finding the child unresponsive in her crib.
A further investigation revealed O'Connor was at home and responsible for the child's care and that the child allegedly had access to methadone in the home, officials said.
On July 22, O'Connor was charged and taken into custody.
"While the death of this baby is tragic, the investigation remains active and ongoing and additional charges may be forthcoming," Prosecutor Bradley Billhimer said.
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