His death, at the Federal Medical Center at the Butner Federal Correctional Complex, was confirmed to The Associated Press by Ed Ross, a spokesman for the Federal Bureau of Prisons. No cause was given, but Mr. Calabrese was known to be in poor health, with heart disease and other ailments.

Mr. Calabrese, an especially vicious member of the Chicago organized crime family known as the Outfit, was serving a life sentence after his conviction, with four other men, on racketeering charges in September 2007 in what became known as the Family Secrets trial.

The trial was the result of years of federal investigation aimed at weakening the Outfit and clearing 18 unsolved murder cases dating to the 1970s. A jury found Mr. Calabrese the perpetrator of seven of the murders, but at a sentencing hearing in January 2009, Judge James Zagel of Federal District Court, by his own reading of the evidence, held him responsible for six additional killings.

The trial was a sensational one, with testimony for the prosecution from Mr. Calabrese's closest relatives. His brother, Nick, an admitted gangster himself, described his brother's murderous style: He beat his victims, strangled them with a rope and then cut their throats.

And his son Frank Jr., who had once served time in the same prison with his father, told how he had volunteered to cooperate with the F.B.I., writing a letter from his cell, and had recorded conversations with his father in which he spoke about mob killings. He had wanted his father to back away from the mob, Frank Jr. said, explaining his decision, and his father had promised to do so but had not.

At the sentencing hearing, another son, Kurt, testified that his father had beaten and threatened him, prompting Judge Zagel to remark, "I've never seen a case in which a brother and a son — and counting today, two sons — testified against a father."

The judge added, speaking to the defendant, "I just want to say that your crimes are unspeakable."

Mr. Calabrese was born on the West Side of Chicago on March 17, 1937. According to his testimony at his trial, he attended several grade schools, fared poorly and eventually discovered he was dyslexic. He sold newspapers on the street as a boy, but grew up to become a thief. He said that he was in the Army after the Korean War but that he deserted.

By the early 1960s he was running his own loan sharking business when he was approached by Angelo LaPietra, a rising member of the Outfit who became his mentor. Mr. Calabrese never denied loan sharking, but he claimed at his trial that he had nothing to do with any murders.

Mr. Calabrese was married twice and had three sons, Frank Jr., Kurt and Nicky. Information about survivors was unavailable on Thursday.

Frank Calabrese Jr. wrote about his family, his own criminal activities and his part in convicting his father in a 2011 book, "Operation Family Secrets: How a Mobster's Son and the F.B.I. Brought Down Chicago's Murderous Crime Family," written with Keith and Kent Zimmerman and Paul Pompian.

"There were many people on the streets of Chicago's South Side who played by my dad's rules," Frank Jr. wrote. "But if you crossed Frank Calabrese he was fast and furious. My father had multiple personalities, and what made it hard was that I never knew which one I might be dealing with at any given time. He was a chameleon and could change in an instant. A lot of people knew about his dual personality, but only a tight core knew about his third, the deadly one.

"The first was the caring and loving provider, the patriarch. The second was the controlling and abusive father, demanding and strict, the streetwise Outift member who ran a vicious and profitable crew. And the third was the killer, whose method of murder was strangulation, followed by a knife to the throat."