MIAMI—For years as a federal prosecutor in New York, Daniel R. Alonso led teams that had to search through a maze of anonymously owned corporate entities to expose criminal activity. “It required all kinds of shoe-leather investigating to identify who was really behind these shell companies,” recalled Alonso. “You’d have to subpoena bank records and lawyers, as well as human sources, and even then you frequently hit a dead end.” Now, thanks to a watershed overhaul of U.S. money laundering laws, locating the proceeds from foreign bribery, drug trafficking and financing for terrorists could be as easy as a few keystrokes. The new legislation quietly passed by Congress last month after a decade-long fight is the most sweeping banking reform of its kind since passage of the Patriot Act, in the aftermath of the Sept. 11, 2001, terrorist attacks. For the first time, shell companies will be required to provide …