From the link:
The big winners from Prohibition were, of course, the nation's gangsters. The law had only been in operation for an hour when the police recorded the first attempt to break it, with six armed men stealing some $100,000-worth of "medicinal" whisky from a train in Chicago. From the very beginning, criminals had recognised that Prohibition represented a marvellous business opportunity; in major cities, indeed, gangs had quietly been stockpiling booze supplies for weeks. Legend has it that the first gangster to grasp the real commercial potential of Prohibition, though, was racketeer Arnold Rothstein, whose agents had been responsible for rigging the baseball World Series in 1919. Establishing his "office" at Lindy's Restaurant in Midtown Manhattan, Rothstein brought alcohol across the Great Lakes and down the Hudson from Canada, and supplied it – at a handsome profit – to the city's gangsters.