The F.B.I. says it has opened investigations into 14 companies as part of a wider inquiry into the subprime mortgage market.

The agency did not identify the companies, but said that the inquiries are centering on valuation issues.

Bloomberg News reports that at a briefing in Washington, Neil Power, chief of the F.B.I.'s economic crimes unit, said, "We're looking at the accounting fraud that goes through the securitization of these loans. We're dealing with the people who securitize them and then the people who hold them, such as the investment banks."

These potentially criminal investigations come on top of inquiries being conducted by the Securities and Exchange Commission and the New York State attorney general. On Sunday, Jenny Anderson and Vikas Bajaj of the New York Times reported that Clayton Holdings, a firm that examines mortgage quality for many investment banks, has agreed to provide evidence to the New York attorney general in exchange for immunity.

That development and the new push by the F.B.I. get at the main question about the collapse of the subprime mortgage market: whether Wall Street, which was busy spinning billions of dollars worth of securities and structured products out of a smaller pool of subprime loans, knew how toxic that pool was—and withheld that information from investors.

As Howard Glaser, a mortgage-industry consultant, told the Times, "At the heart of the subprime meltdown is the inability to get information."

The F.B.I. has already been investigating more garden-variety mortgage fraud, mostly cases involving straw buyers and inflated appraisals. It has more than 1,200 cases pending.

But the latest effort raises the stakes considerably.