CHICAGO (CBS) — Former Chicago mob hitman Frank Calabrese Sr. died in prison on Christmas Day, officials said.

The U.S. Bureau of Prisons said Calabrese died in custody on Tuesday. Calabrese, 75, was being held at the Butner Federal Correctional Complex in North Carolina.

READ MORE: Five Cook County COVID-19 Mass Vaccination Sites To Expand Hours In Preparation For Authorization For Children 12 To 15

The North Carolina Office of the Chief Medical Examiner confirmed they have been informed of Calabrese’s death, but had not yet received his body as of Wednesday afternoon.

Calabrese was known as a brutal loan shark. During the Operation Family Secrets mob trial in 2008, he was charged with 13 murders. He was convicted of seven murders and, two years later, was sentenced to life in prison.

As CBS 2’s John Drummond reported at the time of Calabrese’s 2009 sentencing, some of the murders were high-profile, including the 1980 murders of Bill and Charlotte Dauber, who were gunned down on a rural road in Joliet.

READ MORE: Ravinia Festival Announces Lineup For 2021 Season

Calabrese was also convicted of being involved in the 1981 bombing of Hinsdale trucking executive Michael Cagnoni, who was blown to smithereens on an entrance to the Tri-State Tollway.

Federal authorities secretly videotaped Calabrese as he met with friends and mob minions. His brother, Nick Calabrese, was also a key witness against him.

Calabrese’s son, Frank Calabrese Jr. wore a wire and helped land his father in prison. He learned of his dad’s death yesterday and hadn’t been allowed to see or talk to his father since the 2007 trial

“I had a rough period after I left the courtroom and went upstairs because I knew in my heart that was the last time I’d ever see my father alive and so the day finally came and I found out,” said Calabrese Jr.

He was emotional following his father’s passing.

MORE NEWS: 10 Inmates In One Pod Test Positive For COVID-19 At Lake County Jail In Waukegan

“The emotional part for me right now is the good side,” said Calabrese Jr. “The emotional part of me is I know he is in a better place now and that is my feeling, especially with passing away on Christmas Day. I feel that he is in heaven. I always felt that he was probably going to go to hell for the bad stuff he did. My thoughts right now, my true feelings, are that he is in heaven.”