An Associated Press report on Wednesday said the FBI is investigating IndyMac Bancorp for fraud.
AP said it didn't know how long the investigation has been going on.
Federal regulators took control of the Pasadena bank last Friday and reopened it Monday as IndyMac Federal.
The thrift, with $32 billion in assets, was a prolific lender during the housing boom, specializing in so-called Alt-A loans that allowed buyers to borrow with little documentation of their finances. Losses are expected to mount among Alt-A mortgages as more borrowers decide to walk away from residential investment property plunging in value.
IndyMac's (NYSE: IMB) failure was the first bank failure in California since 2003 and is expected to cost the FDIC between $4 billion and $8 billion, based on the regulator's preliminary estimates.
IndyMac was set up by Countrywide Financial Corp. in 1985, but the two companies cut ties in 1997 and became direct competitors.