Hayden Panettiere's Cause of Death Revealed

Hayden Panettiere's cause of death has been revealed ... the actress died from fentanyl and several other drugs.

The Greenville County Coroner, in a report obtained by TMZ, ruled Hayden's manner of death as an accident. She was also found to have the following in her system -- Alprazolam (generic Xanax), a muscle relaxant called Methocarbamol, and Quetiapine ... a drug used to treat bipolar disorder, schizophrenia and major depression.

As we reported ... Hayden died August 16 inside a Greenville, South Carolina, apartment after traveling from L.A. with her boyfriend, Brian Hickerson.

She was later found unresponsive by Brian and his brother Zach. Brian gave her Narcan to try and revive her, and they performed CPR until paramedics arrived.

Her autopsy previously found "no signs of trauma" that would have contributed to her death.

Paramedics performed advanced cardiac life support and chest compressions ... but Hayden could not be resuscitated and was pronounced dead at 2:32 PM.

Shortly after her death, the actress' Los Angeles home was searched by federal drug agents ... with sources telling TMZ the officials were looking for evidence tied to her drug dealer.

We reported earlier ... officials believed at the time of the search that she took an oxy pill laced with fentanyl just before her death.

Meanwhile, her West Hollywood neighbors had long suspected a drug dealer had been dropping off drugs to her home.

Sources told us ... Hayden and her drug dealer sent a "slew of messages" about drugs, something the DEA is focused on in their investigation.

Hayden was best known for starring in "Heroes" and "Nashville," as well as roles in "Remember the Titans" and the 'Scream' franchise.