The FBI has added three major banks - Lehman Brothers, Fannie Mae and Freddie Mac - and the giant insurance company AIG to the list of companies it's investigating over claims they made to investors about the value of their businesses.
All three have failed in spectacular fashion, leading to a $US700 billion bailout proposal in the United States.
It would involve taxpayers' money being used to buy bad loans to prevent any more banks from going under.
The FBI said on Wednesday the probe was in a preliminary stage and no criminal charges were imminent.
The agency is investing 26 cases of potential corporate fraud related to the collapse of the US mortgage lending industry.
Last week, FBI director Robert Mueller told Congress that 24 cases of potential corporate fraud were under investigation.
The FBI is under increasing pressure from lawmakers to investigate fraud related to the mortgage crisis.