Petty is best known as the frontman of Tom Petty and the Heartbreakers, which formed in 1978 and had hits such as "Mary Jane's Last Dance" and "Learning to Fly".
The music legend's family issued a statement saying that Petty had been touring for 53 dates with a fractured hip that developed into a full break.
Petty died October 2 at UCLA Santa Monica Hospital after his family made a decision to take him off life support.
According to reports, Petty's autopsy was released this week to show how he died.
The coroner's office attributed Petty's death at the age of 66 to a "mixed toxicity" of fentanyl, oxycodone, temazepam, alprazolam, citalopram, acetyl fentanyl, and despropionyl fentanyl. His death certificate initially listed his cause of death as "deferred" while an autopsy could be completed.
Fentanyl is roughly 35 times stronger than heroin and is available with a prescription.
Petty suffered from physical ailments such as emphysema, knee problems and he had significantly fractured his hip, his family said. Petty continued to tour despite these limitations, they said.
The family also used the annoucement as an opportunity to talk about the ongoing opioid crisis.
Several other celebrities have died as a result of the opioid epidemic in recent years, including Prince in 2016 and Philip Seymour Hoffman in 2014.
Petty's family said they hoped the revelation would add to the national discussion on opioid abuse.
According to the statement on the musician's website, the coroner found Petty "passed away due to an accidental overdose as a result of taking a variety of medications". Pills found in his home were labeled "Watson 385," a stamp used to identify some hydrocodone-based painkillers, but tests revealed that those pills contained fentanyl instead.
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