Michael Miske, 50, died of "toxicity of fentanyl and para-fluorofentanyl," the medical examiner's office said in a statement. The death appears to have been accidental but the case is still under investigation, it added, and an autopsy report will likely take at least another 30 days.
It's not clear how Miske got ahold of fentanyl or para-fluorofentanyl while at the Honolulu Federal Detention Center. Para-fluorofentanyl is a synthetic opioid that appears in illicit drugs and is more potent than fentanyl.
The Bureau of Prisons did not immediately respond to an email seeking comment.
Miske was found unresponsive at the lockup on Dec. 1. Efforts by staff and emergency medical responders failed to save him, the agency said.
He was convicted in July on 13 charges including racketeering conspiracy, murder in the aid of racketeering, and kidnapping resulting in death. He was accused of orchestrating crimes that included the kidnapping of a 72-year-old accountant who owed a debt, the release of a toxic chemical into a rival's nightclubs and the killing of his late son's best friend.
The conviction entitled the government to take control of up to $28 million in Miske's assets, including boats, houses and artwork.
He was scheduled to be sentenced on Jan. 30.
The Bureau of Prisons operates 122 federal lockups across the country. It has suffered a series of incidents and crises in recent years, from rampant sexual abuse and other criminal misconduct by staff to chronic understaffing, escapes and high-profile deaths.
In August an inmate and two other people were charged with conspiring to mail drugs to a penitentiary in California, where a mailroom supervisor died after opening a letter that prosecutors said was laced with fentanyl and other substances.