Just when they thought the worst of the mortgage crisis was behind them, billions of dollars in bad loans from the debacle may be rising from the dead and creeping back on the balance sheets of the largest U.S. banks. Big lenders including Bank of America, J.P. Morgan Chase and Wells Fargo may be forced to repurchase troubled home loans from insurers and mortgage-finance giants like Freddie Mac that had agreed to take on risks associated with those assets during the real estate boom. The banks are setting aside more reserves to cover the potential costs of such repurchases, cutting into earnings.