A Hawaii crime boss who died in federal detention this month was killed by an opioid overdose, Honolulu’s medical examiner said Tuesday.

Michael Miske Jr., who was convicted in July of operating one of the most notorious organized crime rings in Hawaiʻi, died of a fentanyl overdose in the Federal Detention Center, according to city officials. Miske, 50, was found dead in his cell Dec. 1.

A medical examiner revealed a Hawaii crime boss died of an opioid overdose while in federal detention. Michael Miske, 50, was found unresponsive at the facility on December 1. It was later revealed that he died from "toxicity of fentanyl and para-fluorofentanyl,