The Federal Deposit Insurance Corp. is conducting about 50 criminal investigations of former executives, directors and employees at U.S. banks that have failed since the start of the financial crisis. The agency responsible for dealing with bank failures is stepping up its effort to punish alleged recklessness, fraud and other criminal behavior, as U.S. officials did in the wake of the savings-and-loan crisis a generation ago. More than 300 banks and savings institutions have failed since the start of 2008, but just a few have led to criminal charges being filed against bank officials.