Freddie Mac's investment shop, which officials say is walled off from mortgage insurers, have been trading in securities that bet on homeowners carrying high-interest loans while rules for refinancing have tightened.

A report from National Public Radio and Pro Publica found that the securities traders were buying inverse floaters – securities based on the interest payments on mortgages. The principal portions had been split off for sale to more risk-averse investors. Meanwhile, the mortgage side of Freddie was on the hook for defaults because it guaranteed the mortgages.

The investments, which the report said are legal, left the bankrupt agency with risks on both sides of the transaction if homeowners defaulted. It had to pay the mortgage debt and it lost the interest income.

The report's focus was that by tightening credit rules, Freddie effectively was locking some homeowners out of refinancing higher-interest mortgages that it had stake in keeping in effect.

Pro Publica which publishes only on the Web, describes itself as an investigative journalism organization focusing on stories in the public interest. It is based in New York City and won a 2011 Pulitzer Prize for reporting.