A Weymouth couple were arrested Monday and charged in the drug overdose death of their 4-year-old daughter, Plymouth County District Attorney Timothy J. Cruz announced.

Michael Riley, 34, and his wife, Carolyn Riley, 32, will be arraigned Tuesday in Hingham District Court. They face first degree murder charges for what prosecutors call an "intentional overdose" of their 4-year-old daughter, Rebecca Riley.

In December, police officers responded to the couple's former home in Hull where they found Rebecca Riley dead on the floor in the master bedroom.

An autopsy found the cause of death as a lethal combination of several prescription drugs, including a fatal dose of Clonidine, which Rebecca Riley had been taking for attention deficit hyperactivity disorder, the district attorney's office said.

The coroner's report also found Valproic Acid, also known as Depakote, Dextromethorphan, a cough suppressant in over-the-counter medicines, and the antihistamine, Chlorpheniramine, in Rebecca Riley's system.

She also had been prescribed Depakote and Seroquel to treat bipolar disorder, Cruz's office said. A psychiatrist diagnosed Rebecca Riley with attention deficit hyperactivity disorder and bipolar disorder when she was 2 years old.

The couple is innocent, their mothers told The Boston Globe.

Carolyn Riley's mother, Valerie Berio, said the charges are "ludicrous."

"She was their treasure, their angel," Berio told the Globe. "They loved her more than life itself. They didn't consciously give her anything to make her go to sleep and not wake up."

Bridget Norton Middleton, a spokeswoman for Cruz, said she did not know if the Rileys had hired a lawyer.

No telephone listing could immediately be found for the Rileys.