BOSTON -- A Hull couple was arrested in connection with the death of their 4-year-old daughter, Plymouth County District Attorney Timothy J. Cruz said Monday.

Carolyn Riley, 32, and Michael Riley, 34, were arrested Monday and charged with the first-degree murder of their daughter, Rebecca.

Hull police responded to a report of an unresponsive child at the family's home on Dec. 13, 2006. Police said when they arrived at the Lynn Avenue home, they found Rebecca, 4, dead on the floor of her parents’ bedroom.

An investigation by the State Police Detective Unit and the Hull Police Department found that Rebecca was being prescribed several medications at the time of her death -- Clonidine for attention deficit hyperactivity disorder, Valproic acid and Seroquel for bipolar disorder. Rebecca was diagnosed with both of the disorders at 2 ½ after being taken to a psychiatrist by her mother, Cruz said.

The Office of the Chief Medical Examiner determined that the cause of Rebecca’s death was intoxication due to the combined effects of Clonidine, Valproic acid, Dextromethorphan and Chlorpheniramine. Cruz said the girl died after being given an intentional overdose of Clonidine and her death was a homicide.

Carolyn and Michael Riley were arrested on Monday in Weymouth. They will be arraigned Tuesday in Hingham District Court.