FBI said to be probing AIG, Lehman, Fannie Mae and Freddie Mac
FBI spokesman John Miller declined to comment Tuesday.
The law enforcement official with knowledge of the probes, who was not authorized to speak publicly about them and requested anonymity, cautioned that the investigations were highly preliminary.
Two of the investigations were apparently launched this week. At a congressional oversight hearing last week, FBI Director Robert S. Mueller III testified that the bureau had 24 firms under investigation.
The Times has reported that the figure includes several California institutions, including IndyMac Bank of Pasadena, which failed in July at a cost of about $8.9 billion.
The Securities and Exchange Commission also is believed to be assessing possible civil fraud claims against the four entities, the official said.
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".