A toddler discovered unconscious in an apartment in January died of a heroin overdose, authorities said Wednesday, citing autopsy results.

The death of 2-year-old Sherlyn Polonia was ruled a homicide, said medical examiner's spokeswoman Ellen Borakove.

It was unclear how the heroin got into the girl's system. After her death, police said they had found a stash of drugs on a night table where she slept.

The girl's mother and the mother's live-in boyfriend were arrested on charges of drug possession, endangering the welfare of a child and reckless endangerment. The Bronx district attorney's office had no immediate comment on whether the charges would be upgraded.

The Administration for Children's Services, which has come under scrutiny for lapses in how it monitors at-risk children, said it had received no reports of abuse regarding Sherlyn.

A 6-year-old boy living in the home was unharmed, police said.