From a 1968 Report by the Senate Subcommittee on Legal and Monetary Affairs (p. 4-5):
Organized crime derives most of its revenue from gambling….
Organized crime’s next largest source of revenue is from “loan sharking” or “shylocking,” that is the lending of money at highly usurious rates of interest, generally in circumstances where the lender is more interested in perpetuating interest payments than in collecting principal; and where force and threats are used to effect collections, to eliminate protests when interest rates are raised, and to prevent the beleaguered borrower from reporting the activity to law enforcement officials.
The President’s Crime Commission indicated that no reliable estimates exist of the gross revenue that racketeers derive from organized loan sharking but that profit margins are higher than from gambling operations. Many law officials classify the business in the multibillion-dollar range. Gambling profits often are used to “bankroll” loan sharking operations.
Other major sources of organized crime’s profits are from trafficking in narcotics…; extortion; prostitution; bootlegging; fraudulent bankruptcies-“scam” operations–in which merchandise is quickly dumped at low prices without paying creditors; in fact, the whole gamut of criminal endeavor.
(Subcom on Legal and Monetary Affairs, Committee on Government Operations. Senate. “Federal Effort Against Organized Crime: Report of Agency Operations. June 1968).
(I gave the link I used but I’m not sure if it is freely available.).