Parents accused of killing their 4-year-old daughter with an overdose of prescription drugs had concocted symptoms of mental illnesses to in a bid to qualify the girl for government benefits, a prosecutor said Tuesday.

Michael and Carolyn Riley pleaded not guilty to first-degree murder charges Tuesday and were ordered to remain in custody without bail.

The Rileys' applications for Supplemental Security Income for their daughter, Rebecca, were twice rejected after government doctors examined her and found no evidence to back the parents' claims of bipolar disorder and attention deficit hyperactivity disorder, or ADHD, Assistant District Attorney Frank Middleton said.

Rebecca was found dead on the floor of her parents' bedroom on Dec. 13. A medical examiner said she died of a lethal combination of prescription drugs, including a fatal dose of Clonidine, which she had been taking for ADHD.

The Rileys' attorneys blame the girl's death on her psychiatrist.

"The medicines that a totally irresponsible doctor gave her killed her -- not the parents," said John Darrell, Michael Riley's lawyer.

Rebecca's older siblings, now ages 11 and 6, already had gone to the psychiatrist, Dr. Kayoko Kifuji, and were diagnosed with the same disorders and were receiving Supplemental Security Income, the program administered by the Social Security Administration for disabled children.

Middleton said Carolyn Riley told Kifuji that Rebecca had "mood swings" and was "driving me crazy." Kifuji diagnosed her with bipolar disorder at age 3.

Carolyn Riley "continued to feed Dr. Kifuji fabricated symptoms," Middleton said. He said Rebecca's teachers, a school nurse, mental health therapist and neighbors and adults who lived with the Rileys all told a grand jury that "Rebecca showed none of these behaviors."

Prosecutors also say that in one year Carolyn Riley got over 200 more pills than should have been prescribed for Rebecca by claiming she either lost or ruined bottles of pills, and by telling a pharmacy she had run out.

According to a state police investigator's report, witnesses told police the Rileys gave their daughter large doses of powerful prescription drugs to keep her quiet and sleeping for long periods.

The couple's other two children had been diagnosed with the same illnesses and were on almost identical prescriptions.

Kifuji agreed to stop practicing medicine until the state Board of Registration in Medicine completes an investigation. Her attorney, J.W. Carney Jr., has said she did nothing wrong and did not overprescribe medication for Rebecca. He declined to comment Tuesday.