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.