Hayden Panettiere Died From Fentanyl-Laced Oxy, Sources

Hayden Panettiere took one fatal pill the day she died, and multiple law enforcement sources believe it was oxycodone laced with fentanyl.

Our sources -- all connected to the case and with direct knowledge -- say Hayden had a "long time drug dealer" in Los Angeles, who was providing her several black-market prescription drugs, including oxycodone.

We're told Hayden took several drugs she purchased from her dealer on the flight from L.A. to South Carolina the day before she died. Authorities believe she took an oxy the morning of her death, and they think that pill was laced with fentanyl.

We're told the DEA in Los Angeles is actively piecing together the relationship between Hayden and her drug dealer, attempting to trace the fatal pill back to him.

The case has shades of Mac Miller, where a drug dealer was convicted of supplying the rapper with drugs -- including some laced with fentanyl -- that killed him.

Our law enforcement sources caution ... everything hinges on the toxicology results which are pending, but their working theory is that a fentanyl-laced oxy pill killed Hayden.

As we reported, when first responders arrived at the condo where Hayden and her boyfriend, Brian Hickerson, were staying, he gave them a bag of her prescription meds. It would be telling if some of those pills were fentanyl-laced.

Brian administered Narcan to an unresponsive Hayden before paramedics arrived, to no avail.