Now-defunct IndyMac Bancorp Inc. is under investigation by the FBI for possible fraud in connection with home loans made to risky borrowers, The Associated Press has learned.

It was not immediately clear how long the FBI's probe of the bank has been ongoing.

The investigation is focused on the company — which was taken over last Friday by the FDIC — and not individuals who ran it, a law enforcement official said Wednesday. The official spoke on the condition of anonymity because he was not authorized to speak publicly about the investigation.

IndyMac Bank's assets were seized by federal regulators after the mortgage lender succumbed to the pressures of tighter credit, tumbling home prices and rising foreclosures.

The bank is the largest regulated thrift to fail and the second largest financial institution to close in U.S. history, regulators said.

The Office of Thrift Supervision said it transferred IndyMac's operations to the Federal Deposit Insurance Corporation because it did not think the lender could meet its depositors' demands.

FDIC spokesman David Barr could not immediately be reached for comment Wednesday.

Shortly after the FDIC took over operations, Barr said most depositors were given immediate access to up to $100,000 in their accounts and 50 percent of any money beyond that threshold. Depositors with joint accounts or retirement accounts could immediately withdraw greater sums.

Depositors were given receivership certificates for any money they couldn't immediately withdraw and may be able to receive some of that money after the bank's assets are sold off.

Early this week, hundreds of worried IndyMac customers lined up out of the bank's headquarters branch in Pasadena, Calif., demanding to withdraw as much money as they could or get answers about the fate of their funds.

Over the last year, and faced with a cratering housing market, the FBI has opened a wide-ranging probe of companies across the financial services industry, from mortgage lenders to investment banks that bundle home loans into securities sold to investors.

Countrywide Financial Corp., the nation's largest mortgage lender, is also under scrutiny.

Additionally, two former Bears Stearns managers were indicted last month on conspiracy and securities and wire fraud charges alleging they lied to investors in a hedge fund that tanked last year as the subprime market collapsed. Those charges marked the first criminal charges to arise on Wall Street from the subprime mortgage debacle.

In all, the FBI is now investigating 21 companies tied to the subprime mortgage crisis, up from 19 last month. Authorities are looking into at least 1,400 mortgage fraud cases nationwide, and more than 400 real estate industry players have been indicted since March.

FBI Director Robert Mueller has said the investigations focus on accounting fraud, insider trading, and failure to disclose the value of mortgage-related securities and other investments. Losses homeowners and other borrowers are estimated at over $1 billion.