Generations of Americans assume that Al Capone was responsible for the St. Valentine's Day Massacre, the execution-style slaying of seven associates of rival mobster George "Bugs" Moran in a Chicago garage on February 14, 1929. In fact, Capone, the flamboyant Chicago crime boss, was never even questioned in the murders. No one was ever tried in the case, leaving the St. Valentine's Day Massacre the most spectacular unsolved crime in gangland history.

What's not in dispute is that a black Cadillac pulled up to the SMC Cartage Company garage on North Clark Street around 10:30 a.m. and disgorged four—maybe five—men, two wearing police uniforms. They ordered the seven men inside the garage to line up facing the wall and opened fire with two Thompson submachine guns, strafing the victims with more than 70 bullets. Moments later, the gunmen walked out with their hands up, prodded along by the purported cops and drove away. Six of the victims died at the scene. Before succumbing, the seventh sputtered "Cops did it," or "Nobody shot me," according to varying accounts.

As newspapers splashed the gruesome photos across the country, dozens of theories emerged. Most were as riddled with holes as the blood-splattered garage wall. Several key questions remain unanswered:

Early theories held that the seven victims—mob accountant Adam Heyer, second-in-command Albert Kachellek; nightclub owner Albert Weinshank, enforcers Peter and Frank Gusenberg, mechanic John May and Reinhardt Schwimmer, an optometrist who liked to hang out with them—were there to split up a hijacked shipment of whiskey or were heading to Detroit to pick up more. But most were dressed in expensive suits, not the typical booze-unloading attire. They might have been summoned to a business meeting, real or phony, and may have lined up compliantly, assuming corrupt cops were just putting on a show. The victims still had thousands of dollars in their pockets, so robbery apparently wasn't a motive.

Bugs Moran wasn't among them. A lookout for the shooters apparently mistook one of the other men for Moran, who was still a block away, and signaled for the hit to begin too early.

Capone had an airtight alibi. He was in a Dade County, Florida courthouse that morning being grilled by prosecutors about another murder. Of course, Capone could have ordered the hit from Florida. He had motives: His men and Moran's had exchanged gunfire for years and were jockeying for control of bootlegging, cleaning operations and dog racing in and around Chicago. Police did arrest several Capone soldiers, but released them for lack of evidence.

Mob historians offer conflicting theories. In their 2004 book, The St. Valentine's Day Massacre, William J. Helmer Jr. and Arthur J. Bilek assert that Capone tapped a secret St. Louis crew dubbed the "American boys" to eliminate Moran. Not knowing what he looked like, the gunmen shot everyone in the garage to be sure. But Jonathan Eig, in his 2010 biography Get Capone, argues that if Capone wanted Moran dead, he would have stationed a lone hitman outside the rival's house. "To kill seven guys in a carefully planned way—it doesn't make sense," Eig told HISTORY.com.

A possible break in the case came in late 1929, when Fred "Killer" Burke, a sometime-Capone associate, fatally shot a police officer in Michigan. A huge cache of weapons found at Burke's home included two Tommy guns that matched bullets from the St. Valentine's Day hit. Burke got a life sentence for the cop killing, but was apparently never questioned about the massacre guns.

In January 1935, the Chicago American newspaper declared the case "solved," reporting that Bryon Bolton, part of another gang, had confessed to participating in the massacre. Local and federal officials vehemently denied the story, which fizzled out amid the confusion. Behind the scenes, however, the feds did question Bolton, according to a 1936 summary in FBI files. Bolton claimed he purchased the Cadillac and was present at a Wisconsin resort where Capone planned the hit. He also named five triggermen, including Burke and other "American boys," all of whom were dead, missing or in prison on other charges at that point. The FBI apparently never shared that information with Chicago police, and Bolton's claims went uninvestigated.

Still another massacre theory turned up years later in unsealed FBI files. In a 1935 letter, Frank Farrell, a state highway employee, urged FBI Director J. Edgar Hoover to investigate the fatal 1928 shooting of William Davern, Jr., a Chicago police sergeant's son. According to Farrell, Davern told his hoodlum cousin, William "Three-Fingered Jack" White, that Moran's enforcers had shot him. White then arranged the massacre for revenge, Farrell wrote, luring Moran's men to the garage on the pretext of cutting them in on a factory holdup.

Eig says Farrell's account ties up many loose ends, including why the men were in the garage and why Chicago police might have looked the way—or even participated, avenging the loss incurred by one of their own. Skeptics note that White was in prison from 1926 through July 1929. But Eig says he might have bribed his way out for brief periods. White, who also worked an FBI informant, was fatally shot at his home in 1934.

Organized crime was rampant in Prohibition-era Chicago, which boasted 10,000 speakeasies and 500 unsolved gangland murders, according to Helmer and Bilek. "The whole system was corrupt—they'd been bought off at every level," Eig said. Capone was arrested frequently, but never held for long. Bolton told the feds that Chicago's chief of detectives was on Capone's payroll, getting $5,000 a week at the time of the massacre.

By contrast, when Philadelphia police arrested Capone and a bodyguard later in 1929 for carrying illegal weapons, they were charged, convicted and sentenced to a year in prison in less than 16 hours.

Although J. Edgar Hoover later built the FBI into a crime-fighting powerhouse, he wanted no part of the St. Valentine's Day Massacre, insisting it was a local matter. He brushed off Justice Department requests to investigate the case and famously denied that organized crime even existed well into the 1950s. Several writers have suggested that Mafia dons had blackmail material on the FBI chief; others contend he was simply more concerned with bank robbers and suspected subversives.

Determined to stop Capone, President Herbert Hoover (no relation) ordered his Treasury Department to investigate the gangster’s finances instead. The man dubbed "Public Enemy No. 1" was sentenced to 11 years in prison for federal tax evasion in 1931.

Could Capone have been charged in the massacre after his release, as some newspapers speculated? Conceivably, but by then Capone's brain was so addled by syphilis he was deemed no longer a threat to society. He died in 1947.

Besides, Eig suggests that letting the cloud of suspicion hang over Capone was more useful to federal authorities than trying to prove it in court and risking him beating the rap again. Meanwhile, Capone's legend grew ever larger in books, movies and TV series as the mobster who got away with the St. Valentine's Day Massacre.