HONOLULU (AP) — A Hawaii crime boss who was convicted of racketeering charges last summer has died in prison, authorities said.

Michael Miske, 50, was found unresponsive Sunday at the Federal Detention Center Honolulu and life-saving efforts by staff and emergency medical responders failed to save him, the Bureau of Prisons said in a news release. It was unclear what caused his death.

His trial attorneys didn't immediately respond to a Monday email seeking comment.

Miske was convicted in July of 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.

Although officials haven't released Miske's cause of death, recent investigations have highlighted concerns within the federal prison system.

A Justice Department watchdog report that was triggered in part by high-profile deaths such as those of notorious gangster Whitey Bulger and financier Jeffrey Epstein found policy violations and operational failures contributed to the deaths of hundreds of federal prisoners over the years.

An ongoing Associated Press investigation has uncovered deep, previously unreported problems within the Bureau of Prisons, including rampant sexual abuse and other staff criminal conduct, dozens of escapes, chronic violence, deaths and severe staffing shortages that have hampered responses to emergencies, including inmate assaults and suicides.