By Randall Mikkelsen

It was not immediately clear how long the FBI's probe of IndyMac has been underway. The bureau said on Wednesday it had raised to 21 from 19 the number of corporate targets in its investigation of the mortgage industry.

U.S. banking regulators seized mortgage lender IndyMac on Friday after withdrawals by panicked depositors led to the third-largest banking failure in U.S. history.

The FBI declined to comment on IndyMac, and law enforcement officials spoke about the bank on condition that they not be identified.

Evan Wagner, a spokesman for IndyMac said: "The first we heard of the investigation was in the press accounts today. We're not in a position to confirm or deny (the investigation) because it's an uncorroborated press account."

He added: "It shouldn't be surprising that there is an investigation because of the size of the bank failure."

Frank Sillman, chief executive officer of IndyMac Mortgage Bank, was not immediately available for comment.

"The FBI currently has 21 investigations related to the subprime market industry. We receive information from a variety of sources on a daily basis, and we have an obligation to review each allegation on its merits," spokesman Jason Pack said when asked about the bank.

"Given the volatility of today's subprime market, we have seen an increase in subprime related complaints. To protect the integrity of our cases, we do not confirm or comment about specific companies that may or may not be a part of our investigations," he said.

Bureau spokesman Richard Kolko, asked whether IndyMac was under investigation, noted the expansion of the industry probe but said he had no names.

David Barr, assistant director, office of public affairs for the Federal Deposit Insurance Corporation said the FDIC is not commenting.

The bureau has publicly acknowledged only one company -- Doral Financial Corp -- as involved in its corporate probe of the mortgage industry. The largest U.S. mortgage lender, Countrywide, also is under FBI investigation, authorities have said, although the FBI has declined to comment and Countrywide said it was unaware of any investigation.

When the FBI disclosed its industry investigation, major investment banks Goldman Sachs , Morgan Stanley and Bear Stearns Cos each said the government had asked them for information, but there was no confirmation of any FBI role. Beazer Homes said last year it had received a federal grand jury subpoena related to its mortgage business.

In its industry probe the FBI is looking at whether fraud was committed in areas including securitizing loans, insider trading and whether firms properly disclosed the value of their assets.

Pack also said he was unaware of any new stock manipulation probe being launched in the wake of Fannie Mae and Freddie Mac problems.

The U.S. Securities and Exchange Commission issued an emergency rule on Tuesday to limit certain types of short selling in major financial firms, including Fannie and Freddie.

(Additional reporting by James Vicini and by Jennifer Martinez in Los Angeles; editing by Carol Bishopric)