A 34-year-old woman who died in NYPD custody on Staten Island after being revived from an overdose in December was killed by a deadly combination of heroin and a designer version of a prescription painkiller, the city medical examiner ruled.

Nicole Garbellotto died Dec. 8 of an accidental overdose of furanyl fentanyl — a designer "analog" of the painkiller fentanyl — heroin and alprazolam, the was generic name for Xanax, Julie Bolcer, a spokeswoman for the city medical examiner's office said Wednesday.

Garbellotto's family sued the city for information behind the circumstances of her death.

Police found her passed out Dec. 7 behind the wheel at the Verrazano Bridge toll plaza. FDNY medics revived her with a shot of the anti-overdose drug naloxone — then took her to Staten Island University North Hospital.

She was released into NYPD custody the, and was found unconscious inside her cell at the 120th Precinct stationhouse in St. George the next day.

It's unclear how long she was in the stationhouse.

She died at Richmond University Medical Center around noon Dec. 8.