By Randall Mikkelsen

WASHINGTON (Reuters) - Failed bank IndyMac is under investigation by the FBI for possible fraud involving its mortgage lending, unnamed law enforcement officials said on Wednesday.

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.

"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.

The Federal Deposit Insurance Corporation, which took over IndyMac, did not immediately return calls seeking comment.