Chicago mobster Frank Calabrese Sr. died in federal prison in North Carolina on Tuesday at the age of 75, authorities said.

Calabrese, who was sentenced in 2009 to life in prison, died on Christmas Day at the Butner Federal Medical Center, said Ed Ross, a spokesperson for the Federal Bureau of Prisons. The cause of death has yet to be revealed but ABC News noted that Calabrese said at his sentencing three years ago that he suffered from different ailments, one being an enlarged heart.

Calabrese was among five men convicted in September 2007 in the Family Secrets trial, which was the result of a multiyear effort by the government to bring down a Chicago organized crime family that called themselves Chicago Outfit. The investigation aimed at clearing 18 unsolved mob murders that happened as early as the 1970s and though none of the defendants in the trial were charged with murder, according to the news report, the jury found Calabrese and two others responsible for killings designed to silence witnesses as a way to carry out mob vengeance.

"It's very emotional right now because there were two sides to my dad, and I miss the good side," Calabrese's son Frank Calabrese Jr. told the Chicago Sun-Times after his father's passing. "I believe he was taken on Christmas Day for a reason. I hope he made peace. I hope he's up above looking down on us. ... He's not suffering anymore. The people on the street aren't suffering anymore."

He helped put his father behind bars by secretly recording for the FBI his father boasting about his mob killings. The late Calabrese's brother also testified against him, saying he preferred to strangle his victims and then slash their throat as a method to make sure they were dead.

The late mobster started by providing loans in the 1950s on behalf of the mob to people in Chinatown, according to WebProNews. He and his sons were arrested in 1995 for using threats and violence to enforce a loansharking scheme. He was sentenced to 118 months in prison in 1997.