The US government has stepped in to rescue some struggling institutions
The FBI has begun an investigation into four major US financial institutions caught up in the current financial crisis, US media say.
Investigators are reportedly examining possible fraud by mortgage giants Fannie Mae and Freddie Mac, the failed bank Lehman Brothers and insurer AIG.
Top managers at those firms are also being investigated, the reports say.
In the last year, as the US housing market slumped, the FBI began a broad inquiry across the financial sector.
It was prompted by concerns over the way high risk, "sub-prime" mortgages were being sold.
The FBI has been looking at lenders who sold home loans to buyers on low or unpredictable incomes, and also the investment banks that packaged these loans and sold them on.
Bankruptcy
Investigations into the four companies were at an early stage, officials told the Associated Press news agency.
ABC News, citing unidentified sources, said the probes were assessing whether company officials systematically misled investors about the financial strength of their institutions.
The slump in the US housing market has resulted in billions of dollars of losses for these banks and turmoil in world credit markets.
Freddie Mac, Fannie Mae and AIG are all being bailed out by the US government following bankruptcy fears.
Lehman Brothers, a top investment bank, has filed for bankruptcy protection.
Last week FBI Director Robert Mueller said more than 20 large financial firms were already under investigation.
The FBI is investigating 26 firms and 1,400 individuals involved in the US financial crisis for fraud and "sub-prime lending practices". Freddie Mac, Lehman Brothers, Fannie Mae and AIG are among the firms being scrutinized after recently receiving federal bailouts. 

Investigators, who are cooperating with the IRS, Postal Office, and other federal offices to complete their investigation, are concerned that major corporations may have also forced or bribed ratings agencies to favor them.

The probe, which is in early stages, began eight months ago when the FBI began taking a close look at the mortgage industry and widespread irresponsible loaning practices. At least one corporation has been raided, but so far no evidence of fraud has been found.

The FBI has questioned executives of each of the firms closely, and arrested two in June. An anonymous source told The Times that the firms had been ordered to "hold all papers and e-mails under lock and key" as the FBI scours the finances of each firm. 

Many of the companies and individuals being investigated are at the center of the nationwide financial crisis and controversial bailout plans, and have been widely blamed for the country's financial crisis. The investigation has come at a time when the eyes of many in the US and around the world are turned towards the financial markets, as Congress and politicians scramble to fix the crisis while the election date looms closer. 

Officials told CNN that it would be a long time before the investigations were finished, adding a warning: "Don't expect indictments tomorrow or next week or next month".