Robert Dettmering, 76, pleaded no contest on January 20 and was convicted of conspiracy to commit first-degree intentional homicide.
The Department of Corrections confirmed the death, saying Dettmering died Thursday evening and they have started an internal investigation.
Prosecutors say Dettmering offered a man $10,000 to kill his son and his son's wife in July of last year.
The man instead called the Marinette County Sheriff's Office and the Wisconsin Department of Justice sent an agent posing as a hit man to meet at Dettmering's bar, Kranky Bob's. Dettmering was arrested when he offered the agent $1,000 as a down payment on the murders.
Dettmering said he was upset his son was selling property he had given him.