The four firms -- Fannie Mae, Freddie Mac, American International Group Inc. and Lehman Bros. Holdings Inc. -- are among 26 entities that the FBI is scrutinizing for their role in the mortgage meltdown.
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.