The parents of a New Jersey infant who died of heroin intoxication in 2008 have pleaded guilty to manslaughter after admitting they rubbed heroin on the baby's gums because he was teething.

Just how the 11-month-old boy known as "Baby JM" ingested the heroin had remained a mystery until investigators in Ocean County got new information in the Seaside Heights case in October.

Ocean County prosecutors said Thursday that 35-year-old Denise Manco and 37-year-old Rondell Moore entered guilty pleas earlier this week to one count of second-degree manslaughter. They face up to 10 years in prison at sentencing, which is set for Feb. 22.

Authorities say both defendants admitted placing heroin in the baby's mouth and rubbing it on his gums because he was teething.