Who killed infamous mob boss Sam Giancana? Hitman unmasked by NBC Chicago

"Who killed Momo" has been one of Chicago's most nagging crime questions for five decades, since the hoodlum's savage death in late spring of 1975.

The identity of Sam Giancana's murderer was long considered the Chicago Outfit's greatest hit in a city known for its bloodthirsty gangsters and ruthless butchery.

But as NBC Chicago investigated the mob's last family secret, our team found that the identity of Giancana's assassin was no secret in some circles.

Federal investigators say Giancana's killer was unmasked long ago.

June 19, 1975, was unusually hot and sticky, and just before 11 p.m., under a moonlit sky, Chicago mob magnate Sam Giancana had just allowed a guest into the basement kitchen at his Oak Park home.

As Giancana fried up a late-night sausage snack, the visitor shot him several times in the head. Giancana was dead on the floor.

New Chicago FBI Special Agent in Charge Douglas DePodesta said the agency "absolutely" considers Giancana's killing to be an unsolved, open case.

"What it looks like is we are always seeking for information from the public so we can follow up," said DePodesta.

It is a similar approach at west suburban Oak Park police headquarters. Giancana's investigative file No. 75-07161 is not public because the case is still officially unsolved.

NBC Chicago has obtained copies of police records from an outside source that reveal the gun used to kill Giancana was purchased in South Florida. The .22 pistol was one of a pair bought at the same time at a Miami gun shop. One handgun was used to execute a Milwaukee mobster, and the other was used on Chicago Outfit icon Sam Giancana.

"That's an open investigation, so I can't talk to the details of that case," said the FBI's DePodesta.

NBC Investigates asked DePodesta, "In your mind, though, do you believe that the bureau knows how that happened?"

His answer: "No," the bureau doesn't know.

But 1,750 miles to the west, in Las Vegas, the case is considered closed for federal witness Frank Calabrese, Jr., who says Giancana's killer has been known to federal investigators and prosecutors for decades.

The Chicago mob defector spoke about his former life in the Outfit to packed rooms daily at The Mob Museum in Vegas, a place where Giancana's mug has a prominent position, from an era when he and the Chicago Outfit controlled casinos for decades.

"You know, there's a lot of these mob mysteries that we'll probably never solve, and the Giancana one is definitely near the top of the list," said museum exhibits director and mob expert Geoff Schumacher.

Calabrese contends there is no mystery.

His Chicago mob boss father Frank Sr. and Chicago mob hitman uncle, Nick, provided the answer to who killed Momo Giancana years ago.

"My information that I got from my dad verbally was that Tony Accardo killed Sam Giancana. My dad said that it was Tony that killed him," said Calabrese, Jr. 

Tony Accardo, aka Anthony Accardo, was nicknamed JB or Joe Batters for his fondness of baseball bat beatings.

Nobody outranked the 69-year-old Accardo in the Chicago Outfit back then. Sending a mob legend such as Giancana to his death would have required Accardo's personal approval. But Giancana was actually killed by Accardo, according to mob hitman Nick Calabrese in an FBI interview newly obtained by NBC 5 Investigates.

The FBI interview report concludes, "Accardo, nicknamed JB, was involved in the homicide of Giancana." And then the reports cite one mob boss as saying, "you're never too old to work like JB."

But there's more. The Momo murder weapon was found in a roadside forest preserve located between Giancana's house and Accardo's home in River Forest.

The FBI had Accardo's house under surveillance in 1975. Records obtained by our team document comings and goings from the Accardo house and reveal that one of the capo's cars left before Giancana was killed and returned home afterward.

"He goes alone to Sam's house and they're going to talk about the upcoming Senate hearing commission…" recalled Calabrese, Jr.

Mobwatchers suggest Accardo, Al Capone's one-time bodyguard and driver, was concerned that Giancana was a loose cannon and might spill mob secrets to the U.S. Senate committee that had summoned him.

Calabrese, Jr., focuses on the fear that Giancana sowed in Accardo and his fellow outfit bosses. "Sam was the one that could really get Tony in a lot of trouble."

Thirty years after Giancana was murdered, Calabrese, Jr. and his assassin-uncle Nick famously became government witnesses in an FBI undercover investigation known as Operation Family Secrets.

Based on their information, prosecutors publicly cleared 18 Outfit murders and convicted more than a dozen major Chicago mob figures.

But NBC 5 Investigates has been told by four top federal investigators who worked on the landmark case that the government actually solved 30 mob killings, including Giancana's. The law enforcement consensus, they say, is that Tony Accardo killed Giancana.

FBI records state that Giancana's killer will never be prosecuted because he is dead. Anthony Accardo died of natural causes in 1992. He was 86 years old. Giancana and Accardo's final resting places are not far from each other in west suburban Hillside.

According to Outfit experts, the two mob legends only tolerated each other. Until his death, Tony Accardo boasted that he'd never spent a night in jail. Some mobwatchers believe that when Accardo killed Sam Giancana, it was an act that ensured he never would.

Bottom line: Sam Giancana was murdered by a man with the means, motive and opportunity to do it.

Anthony Accardo's means were well-known. He never shied away from ordering executions, according to federal investigators, or lending a personal hand in Outfit killings. His violent crime resume went all the way back to the St. Valentine's Day Massacre in 1929 when Accardo was on the machine gun murder team carrying out Al Capone's marching orders inside a Clark Street warehouse. 

Accardo had motives. He was fueled by lifelong grudges with Giancana, and he was afraid Giancana was a loose cannon who could land him in handcuffs and then behind bars.

And Accardo had opportunity. He was one of the few people who could knock on Giancana's door before midnight and have it opened.

Why did he personally do it and not hand off the hit to an underling? Mobologists suspect that in 1975 Accardo didn't trust anyone else to get it done and wanted to guarantee that the hit wouldn't come back on him … by keeping it secret.

A 50-year secret … now exposed.