The actress Hayden Panettiere, who starred in “Heroes” and “Nashville,” died last month at 36. The authorities said on Tuesday that she died from a toxic mixture of fentanyl, alprazolam, methocarbamol and quetiapine, and ruled her death accidental.

The Greenville County Coroner’s Office said an autopsy found no trauma contributing to the death. Alprazolam, the generic version of Xanax, treats anxiety; methocarbamol, or Robaxin, is a muscle relaxant; and quetiapine, or Seroquel, is an antipsychotic used to treat schizophrenia, bipolar disorder and depression.

On Aug. 16, paramedics responded to an apartment building in Greenville, S.C., where they administered advanced cardiac life support and resuscitation efforts. Ms. Panettiere was pronounced dead at 36.

Ms. Panettiere was with Brian Hickerson, her on-and-off boyfriend who had served jail time for abusing her, at the time of her death. He has not been accused of wrongdoing. A responding officer said Mr. Hickerson showed the officer a bag of medications she had been taking.

Ms. Panettiere had recently written about struggles with drugs, alcohol and chronic pain. Her hairstylist, Erick Orellana, said she had often complained about back pain. Days after her death, the Drug Enforcement Administration joined the investigation.