Illinois Department of Corrections spokeswoman Sharyn Elman said Aleman had been ill and there were no suspicious circumstances around his death.
"He was the hammer of the Chicago mob," said Lee Flosi, a former FBI agent who headed the Chicago office's organized-crime section in the 1990s. "You never want him sitting in the back seat of your car."
Aleman was in prison for the 1972 shotgun slaying of a union official and was suspected in nearly 20 other killings. He was sentenced to serve 100 to 300 years.
The 1997 conviction made legal history in the United States as the first time a criminal defendant had been tried again after an acquittal — the judge had been bought off in his first trial, authorities said.
Aleman was born in 1939 in Chicago. By the early 1970s, he had a reputation as an enforcer for various organized-crime crews, Flosi said. If people engaging in illegal activity neglected to pay tribute to the local mob boss, they would get a visit from Aleman; just the sight of him was often enough to get people in line.
"He had that kind of fear," said Flosi. "He was an intimidating presence."
Aleman gained national notoriety as a cold-blooded executioner.
On the night of Sept. 27, 1972, Teamsters shop steward William Logan was gunned down outside his home on Chicago's West Side as he left for work. Logan was involved in a bitter custody dispute with a relative of Aleman's, but a clear motive for the slaying was never established.
Louis Almeida, an accomplice who testified at Aleman's 1977 trial, said he and Aleman trailed Logan for two weeks, then drove to his house. Almeida said he pulled up as Logan was walking to his car and Aleman called out, "Hey Billy" and shot him with two blasts from a 12-gauge shotgun.
Despite Almeida's testimony and that of a neighbor who said he saw Aleman at the scene, Aleman was acquitted in a bench trial by Judge Frank Wilson. After allegations of a fix surfaced, the case was reopened. Wilson committed suicide in 1990 during the probe, and another trial was ordered despite defense lawyers' objections concerning possible double jeopardy.
At the 1997 retrial, Robert Cooley, a mob lawyer turned government informant, testified that he delivered a $10,000 bribe to Wilson to acquit Aleman.
Aleman was convicted in the second trial. Judge Michael Toomin sentenced Aleman, who had spent 19 years in federal custody on other charges, to up to 300 years.
In 2002, the Illinois Prisoner Review Board denied Aleman parole. The one board member who voted in Aleman's favor was later charged with trading his vote for help in getting his son a gig as an entertainer in Las Vegas. He was later cleared of those accusations.
Flosi, now the owner of a private security firm, said he met Aleman in the early '90s, when his office gathered evidence that eventually led to the hit man's second murder trial.
"He was very cold, very calm, the kind of guy you couldn't get excited. He wasn't going to be intimidated by the badge," Flosi said. "He was a ruthless guy, no doubt about it."