For the most part, banks have said they can’t be called out in court on any of this because they had no idea that so many of these loans went to people who lacked the resources to make even their first mortgage payment.

Wall Street firms were intimately involved in the financing, bundling and sales of these loans, so their Sergeant Schultz defense rings hollow. They provided hundreds of millions of dollars in credit to dubious underwriters, and some even had their own people on site at the loan factories. Many Wall Street firms owned mortgage lenders outright.

Because many of the worst lenders are now out of business, investors in search of recoveries have turned to the banks that packaged the loans into securities. But successfully arguing that Wall Street aided lenders in a fraud is tough under federal securities laws. This is largely a result of Supreme Court decisions barring investors from bringing federal securities fraud cases that accuse underwriters and other third parties as enablers.

Where there’s a will, however, there’s a way. And state courts are proving to be a more fruitful place for mortgage investors seeking redress, legal experts say.

In late June, for example, Martha Coakley, the attorney general of Massachusetts, extracted $102 million from Morgan Stanley in a case involving Morgan’s extensive financing of loans made by New Century, a notorious and now defunct lender that was based in California.

Morgan packaged the loans into securities and sold them to clients, even after its due diligence uncovered problems with the underlying mortgages that New Century fed to the firm, Ms. Coakley said. In settling the matter, Morgan neither admitted nor denied the allegations. Her investigation is continuing.

One of the most interesting aspects of this case “is the active role of state regulators relying upon state law to protect investors,” said Lewis D. Lowenfels, an authority on securities law at Tolins & Lowenfels in New York. “This state focus may well fill a void left by the U.S. Supreme Court’s increasingly narrow interpretation of the antifraud provisions of the federal securities laws as well as the relatively few S.E.C. enforcement actions initiated in this area.”

Last Friday, an investment management firm that lost $1.2 billion in mortgage securities it bought for clients filed suit in Massachusetts state court against 15 banks, accusing them of abetting a fraud. The firm, Cambridge Place Investment Management of Concord, Mass., purchased $2 billion in mortgage securities from the banks, and it says the banks misrepresented the risks in the underlying loans — both in prospectuses and sales pitches.

The complaint says the banks misled Cambridge Place by maintaining that the mortgages in the securities it bought had met strict underwriting requirements related to the borrowers’ ability to repay the loans. Cambridge also contends it relied on the banks’ claims of having conducted due diligence to verify the quality of the loans bundled into the securities.

The complaint also details the anything-goes lending practices during the subprime mortgage boom.

Interviews in the complaint with 63 confidential witnesses turned up such gems as Fremont Investment & Loan, which had been based in California, approving loans for pizza delivery men with reported monthly incomes of $6,000, and management at Long Beach Mortgage, also in California, directing underwriters to “approve, approve, approve.”

One Long Beach program made loans to self-employed borrowers based on three letters of reference from past employers. A former worker said some letters amounted to “So-and-so cuts my lawn and does a good job,” adding that the company made no attempt to verify the information, the complaint stated.

Such tales are hardly shockers. But they provide important context when Cambridge moves up the ladder to the banks that bundled and sold the loans.

For example, the complaint contended that Credit Suisse, from whom it bought $88 million of mortgage securities in 2005 and 2006, told Cambridge of its “superior” due diligence, including a performance review of every loan. Three-quarters of these loans are delinquent, in default, foreclosure, bankruptcy or repossession, the complaint said.

Bear Stearns, now a unit of JPMorgan Chase, sold Cambridge $65 million of securities. It owned three mortgage lenders and told Cambridge it sampled the loans it sold to check underwriting procedures, borrower documentation and compliance, the complaint said.

Among others named in the suit are Bank of America, Barclays, Citigroup, Countrywide, Deutsche Bank, Goldman Sachs, Merrill Lynch, Morgan Stanley and UBS. All of those, as well as Credit Suisse and JPMorgan, declined to comment.

CAMBRIDGE’S lawyers brought its case in Massachusetts under laws barring those who sell securities from making false statements about them or omitting material facts. Jerry Silk, a senior partner at Bernstein Litowitz Berger & Grossmann who represents Cambridge, said, “This case represents yet another example of Wall Street banks’ failure to live up to their basic responsibility to investors — to tell the truth about the securities they are selling.”

Mr. Silk’s firm has jousted with Wall Street underwriters before. In 2004, it recovered $6 billion in a suit against banks that underwrote debt issued by WorldCom, the defunct telecom. Denise L. Cote, the federal judge overseeing that matter, concluded that because investors rely so heavily on underwriters, courts must be “particularly scrupulous in examining the conduct,” she said.

It is too soon to tell if investors will recover losses in mortgage securities. But the efforts are reminiscent of those in the mid-90s against brokerage firms that cleared trades and provided capital to dubious penny-stock outfits such as A. R. Baron and Sterling Foster.

For decades, companies that cleared such trades — Bear Stearns was a big one — escaped liability for fraud at these so-called “bucket shops.” But regulators went after clearing firms by accusing them of facilitating such acts; in a 1999 lawsuit, the Securities & Exchange Commission accused Bear Stearns of enabling a fraud at A. R. Baron. Bear Stearns paid $35 million in fines and restitution to settle the case.

If trust in capital markets is to return, investors must be able to believe what they read in prospectuses. Without that minimum standard, how can Wall Street expect the markets to function again?