United Kingdom, Sept. 14 -- Hayden Panettiere is believed to have died after taking an oxycodone tablet laced with fentanyl.

The Heroes actress passed away last month at the age of 36 and investigations have been ongoing into the circumstances surrounding her death in South Carolina.

And now, multiple law enforcement sources with direct knowledge of the case have told TMZ that Hayden had a "long-time drug dealer" in Los Angeles, who provided her with several black-market prescription medications, and she had taken a number of them with her on her flight from California the day before she died.

Authorities are said to be working on the theory she took the oxycodone pill on the day she died but it was laced with the strong opioid.

Toxic...