Lehman Brothers, AIG, Fannie Mae and Freddie Mac are all now under investigation.
Several US financial firms are under investigation from the Federal Bureau of Investigation (FBI), it has emerged.
The probe represents the latest blow for the American financial system - which has been subjected to volatile markets, forced nationalisations, mergers and the bankruptcy of Lehman Brothers over recent weeks.
Along with the investment bank - whose assets are now being sold off following its declaration of insolvency - insurer AIG and mortgage lenders Fannie Mae and Freddie Mac are all under investigation. The latter three firms have been taken over by the US government, after suffering a loss of investor confidence.
It has been confirmed by a federal law enforcement official that the probe has been launched regarding mortgage fraud, with allegations that executives at the four firms provided "misinformation" to the public. Ongoing FBI investigations into the financial services sector have previously centred on homebuyers being mis-sold high-risk, or sub-prime mortgages by lenders.
The collapse of the sub-prime sector was a key factor behind the onset of the ongoing credit crunch - which resulted in banks becoming less willing to lend to each other and to consumers.
US justice department spokesman Brian Roehrkasse provided few specifics about the investigation, however. "As part of our investigative responsibility, the FBI conducts corporate fraud investigations," he commented.
"The number of cases fluctuates over time, however we do not discuss which companies may or may not be the subject of an investigation."
