WASHINGTON — Sen. Elizabeth Warren (D-Mass.) raised the stakes of her quest to find out why a single Wall Street bank has not been prosecuted in the aftermath of the financial crisis Tuesday, sending a letter to the heads of three federal agencies. Warren, a member of the Senate Committee on Banking, Housing & Urban Affairs asked Attorney General Eric Holder, current Securities and Exchange Commission Chairwoman Mary Jo White and Federal Reserve Chairman Ben Bernanke whether they had done any cost-benefit research into prosecuting a bank versus settling with one, which is equivalent to a slap on the wrist for a profitable financial institution.