The Federal Reserve yesterday auctioned $28.77 billion in safe Treasury securities to big investment firms, part of an ongoing effort to ease credit problems, the Associated Press reported. In exchange for the 28-day loan of Treasury securities, bidding firms can put up more risky investments, including risky mortgage-backed securities and bonds backed by federally guaranteed student loans, as collateral. Bidders’ identities are not made public. To help shore up the shaky student loan market, the Fed agreed last week to let financial institutions put up bonds backed by federally guaranteed student loans as collateral. Yesterday’s auction was the first where that option was available. Spreading credit problems have forced more than 60 lenders to stop making federally guaranteed student loans, either temporarily or permanently.