Child star Hayden Panettiere died from the toxic effects of fentanyl, Xanax and several other drugs in her system, according to an autopsy completed by the Greenville County Coroner's Office in South Carolina.

The coroner's office said Panettiere's death was an accident and that no signs of trauma were discovered that would have contributed to her death.

Panettiere died last month at the age of 36 after she was found unresponsive at a South Carolina residence.

The star of US TV shows Heroes and Nashville had spoken openly about her struggles with substance abuse.

The coroner's office also found methocarbamol - a prescription muscle relaxant - and quetiapine - an antipsychotic medication that is used to treat certain mental health conditions.

Fentanyl is a synthetic opioid that has killed hundreds of thousands of Americans over the past five years alone.

The coroner's office said emergency responders had "initiated advanced cardiac life support measures; however, resuscitation efforts were unsuccessful".

The US Drug Enforcement Agency (DEA) was reportedly joining the local police department's investigation into the child star's death.

Panettiere's ex-boyfriend, Brian Hickerson - who has admitted to abusing her - and his brother Zack were with Panettiere when emergency services came, according to a police report from the day she died.

Just months before her death, Panettiere published a memoir where she discussed at length her issues with addiction and postpartum depression.

Panettiere has said she was introduced to drugs around the age of 16 during a red carpet appearance for Heroes when a member of her professional team gave her something called a "happy pill" for energy.

"I had no idea that this was not an appropriate thing, or what door that would open for me when it came to my addiction," she told People Magazine. "As I got older, the drugs and alcohol became something I almost couldn't live without."

Panettiere's brother, Jansen, died aged 28 from a heart condition. Panettiere revealed in her book that her brother also struggled with substance abuse and used heroin and cocaine leading up to his death.