COLLINGDALE, Pa. (CBS) — A 29-year-old Delaware County woman is being held without bail, charged with the murder of her 11-month-old son in mid-July.

Christine Rivero at first claimed that her youngest son, Niccolo Varner, hit his head in the bathtub while bathing with a five-year-old brother.

Later, she said she went out to snort heroin with friends while the kids were in the tub.

Delaware County District Attorney Jack Whelan says toxicology reports from the coroner astonished him.

“The child’s death wasn’t caused by drowning or suffocation. The child’s death was caused by acute heroin intoxication,” Whelan tells KYW Newsradio.

Whelan can’t say for sure how so much heroin got into the child, but Rivero is facing charges which include third-degree murder.

The five-year-old is with his father, who told investigators that Rivero has had a history of drug problems and he believed she was back on heroin at the time Niccolo died, but Rivero denied it.

She is scheduled for a preliminary hearing next week.