The Federal Bureau of Investigation (FBI), which has already found widespread criminal activity among realtors and appraisers at the lower rungs of the mortgage debacle, is investigating four giant firms and their chief executives at the top of the meltdown for potential mortgage fraud.

Firms reportedly being investigated are Fannie Mae, Freddie Mac, Lehman Brothers Holdings Inc. and the insurer American International Group Inc. Lehman sought bankruptcy protection after the government did not take action to bail it out as it did with the three other entities.

FBI Director Robert Mueller had earlier told Congress that the Bureau is looking at all levels of the mortgage systems and, in the case of the corporate investigations, “the allegations would be there (that) there were misstatements of assets.”

A federal law enforcement official told Reuters the FBI is currently looking at 26 cases of potential corporate fraud related to the implosion of the mortgage industry. Mueller told Congress in mid-September that 24 cases of potential corporate fraud were investigation compared to the 21 cases disclosed by the bureau in July.

Other corporate entities known to be under scrutiny include the failed bank IndyMac Bancorp Inc. and Countrywide Financial Corp., once the nation’s largest mortgage lender, which is now owned by Bank of America.