The Purple Gang - More Than Just a Rhythm Section
Around the same time that Aleister Crowley was attempting to get his mojo on in Detroit, things were heating up in the world of organized crime. Prohibition opened the doors to whole new areas of profit; according to the Detroit News, in 1929 illegal liquor was the second biggest business in Detroit at $215 million a year (just behind automobiles), and there were as many as 25,000 blind pigs operating in the area, including one across the street from police headquarters.
Thanks to a statewide ban on alcohol in 1918, two years before national Prohibition, the Purple Gang got an early start. They also started rather far up the food chain, attacking bootleggers who were attempting to bring smuggled liquor ashore. In later years, they hired on with corrupt union officials as goons in the "Cleaners and Dyers War" (what a great name, huh?), and survived the subsequent trial to become the dominant gang in Detroit. They actually became powerful enough that they managed to keep Al Capone from branching out into Detroit on the basis of threats alone.
As often happens, this reign of terror came to an end because somebody got greedy. A small group of men attempted to strike out on their own, and the gang's response became known as the "Collingwood Manor Massacre". The survival of a single witness to the killings resulted in the rapid decline and eventual end of the gang. Only their reputation survives, and for most people that begins and ends with their knowledge of the lyrics to Jailhouse Rock.
Stories and Speculation
The Purple Gang can serve the same role for Detroit as Al Capone does for Chicago: the all-purpose hook for murders, hidden stashes of contraband, and dark secrets in the backgrounds of centenarian civic boosters.
The Detroit News has a detailed summary of the gang's career. I'd link to some other sites, but (ironically) there are some definite signs of plagiarism on the different sites, with uncited sections of identical material popping up on different websites.