BROCKTON, Mass. – A Massachusetts woman was convicted of second-degree murder Tuesday in the fatal prescription drug overdose of her 4-year-old daughter.

Carolyn Riley, 35, was accused of overmedicating her daughter, Rebecca, on powerful drugs prescribed by a psychiatrist who diagnosed her with attention deficit hyperactivity disorder and bipolar disorder by the time she was 3.

Prosecutors said Riley and her husband, Michael, concocted symptoms of mental illness in their daughter in an attempt to collect federal disability benefits, then overmedicated her for months to keep her quiet and out of their way.

Defense lawyers said Rebecca died of a severe, fast-moving pneumonia, not a drug overdose.

The case reignited debate in the psychiatric community about whether young children can accurately be diagnosed with bipolar disorder and whether they should be treated with powerful adult drugs.

Jurors reached their verdict on the third day of deliberations, rejecting a first-degree murder charge and opting instead for the lesser second-degree murder charge.

Riley showed no emotion as the jury announced its verdict. Superior Court Judge Charles Hely immediately sentenced her to the mandatory term of life in prison, with parole eligibility after 15 years.

Michael Riley faces a separate trial.

Rebecca was found dead on the floor of her parents’ bedroom on Dec. 13, 2006.

A state medical examiner testified that the girl died of a combination of Clonidine, a blood pressure medication the girl had been prescribed for ADHD; Depakote, an antiseizure and mood-stabilizing drug prescribed for bipolar disorder; and two over-the-counter drugs, a cough suppressant and an antihistamine. Rebecca’s uncle and his girlfriend testified that they pleaded with the Rileys to take the girl to a doctor in the final days of her life, when she grew increasingly ill. Instead of getting medical attention for her, Carolyn Riley gave her a lethal dose of Clonidine, prosecutors said.

Defense attorneys portrayed Riley as a loving mother who was struggling to raise Rebecca and her two siblings, who also had been diagnosed with ADHD and bipolar disorder and were taking similar medications. The defense said Riley followed the advice of her children’s psychiatrist, Dr. Kayoko Kifuji, in the amount of drugs she gave her children.