Calabrese Sr. was convicted in September 2007 of carrying out 13 murders related to organized crime in Chicago. The cause of death hasn't been determined but was believed to be natural. The former strongman suffered from multiple ailments including heart disease, the Chicago Sun-Times said Thursday. He was being held at the Butner Federal Medical Center.
Calabrese Sr.'s son, Frank Calabrese Jr. -- who recorded his father bragging about the killings and testified against him during his trial -- said he hoped his father had made peace with his misdeeds.
"It's very emotional right now because there were two sides to my dad, and I miss the good side," Calabrese Jr. said.