Federal prosecutors are trying to thwart the easy access that predatory lenders and dubious online merchants have to Americans' bank accounts by going after banks that fail to meet their obligations as gatekeepers to the United States financial system. The Justice Department is weighing civil and criminal actions against dozens of banks, sending out subpoenas to more than 50 payment processors and the banks that do business with them, according to government officials. In the new initiative, called Operation Choke Point," the agency is scrutinizing banks both big and small over whether they, in exchange for handsome fees, enable businesses to illegally siphon billions of dollars from consumers' checking accounts. The critical role played by banks largely plays out in the shadows because they typically do not deal directly with the Internet merchants. What they do is provide banking services to third-party payment processors, financial middlemen that, in turn, handle payments for their merchant customers. The new, more rigorous oversight could have a chilling effect on Internet payday lenders, which have migrated from storefronts to websites where they offer short-term loans at interest rates that often exceed 500 percent annually. As a growing number of states enact interest rate caps that effectively ban the loans, the lenders increasingly depend on the banks for their survival. With the banks' help, the lenders that typically work with a third-party payment processor that has an account at the banks are able, authorities say, to automatically deduct payments from customers' checking accounts even in states where the loans are illegal.