Dewey probably would have gone after Mangano eventually if Abe Reles not been killed. He was set to go after Anastasia. This was part of the Murder Inc. case. Reles identified him as Anastasia's boss.
But even though Reles's death prevented him to go after Mangano for murder, they could have still tried financial crimes, like tax evasion. Almost every known gangster in the country did have problems with tax agents at the time, even Frank Costello later, despite all his connections. It's interesting by the way that only Al Capone did really find himself in trouble after a tax conviction though (11 years). Most others (not only in Chicago, but everywhere) got off with 1 or 2, rarely 3 years.
There is a book on the subject I had bought, about how the team who brought down Al Capone (not Eliot Ness who played a less prominent role in strictly financial investigations, he wanted to send him down for Prohibition offenses) went to different cities around USA to put in prison local racketeer bosses for financial crimes.
http://www.amazon.com/Money-Trail-Brough...rds=money+trail
