Police in South Carolina have determined that the death of US actress Hayden Panettiere was accidental, bringing their local investigation to a close.

The Heroes and Nashville actor died in August at the age of 36 after experiencing the toxic effects of fentanyl, Xanax and other substances found in her body.

After the medical examiner reported last month that Panettiere had died from a drug overdose, the Greenville Police Department said it had completed its investigation into the case.

The US Drug Enforcement Administration was also said to have taken part in the local inquiry. It remains uncertain whether its separate investigation has ended.

"Based on the Medical Examiner's determination and the findings of the investigation, Hayden Panettiere's death has been ruled accidental," police said in a statement.

Officials, including DEA investigators, had reportedly been examining the events surrounding Panettiere's death.

A Greenville Police Department spokesperson did not provide details about the nature of the DEA's involvement.

Panettiere, who had previously spoken publicly about substance misuse, was found unresponsive at a home in South Carolina on 16 August.

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Last month, the Greenville County Coroner's Office confirmed that the former child performer had died in an accidental overdose involving fentanyl, a synthetic opioid responsible for a major public health crisis in the United States.

The coroner also identified methocarbamol, a prescribed muscle relaxant, and quetiapine, an antipsychotic medication used for some mental health conditions.

The coroner's office said responders had attended a woman in cardiac arrest, while an autopsy found no evidence of trauma that contributed to her death.

Panettiere's former boyfriend, Brian Hickerson, and his brother Zack were reportedly present when emergency crews arrived, according to a police report filed on the day of her death.