As more details unfurl in the Bernard L. Madoff fraud case, so do the lawsuits. And the big accounting firms, which oversaw many of the feeder funds that funneled billions of dollars into what prosecutors describe as the largest Ponzi scheme ever perpetrated, are likely to be among the defendants.

Though Bernard L. Madoff Investment Securities itself was audited by small firms, questions are arising over how major firms like PricewaterhouseCoopers and KPMG overlooked several red flags related to the operations over a number of years. The big accounting firms are likely to face queries about why they gave their seal of accounting to the astoundingly steady positive returns booked by a fund manager whose investment strategy was nearly completely opaque.

One investor in a feeder fund, New York Law School, has already sued BDO Seidman, the auditor of one of its money managers, arguing that the firm failed to notice warning signs related to the $50 billion scandal.

The district attorney for Rockland County, N.Y., Thomas P. Zugibe, has also begun inquiries into Friehling & Horowitz, the three-person accounting firm that provided services to Mr. Madoff’s firm. Many have asked how a company as small as Friehling — a three-employee firm based in New City, N.Y., that occupies a 13-foot-by-18-foot storefront space in an office plaza — could have handled an operation as large as Bernard L. Madoff Investment Securities. Friehling & Horowitz is also the subject of a preliminary ethics investigation by the American Institute of Certified Public Accountants started after the scandal broke.

Another small accounting firm, Sosnik Bell, handled paperwork for investors in Mr. Madoff’s firm, according to Clusterstock, a financial news blog. Sosnik Bell, based in Fort Lee, N.J., processed forms for these investors, and then forwarded its work to the investors’ own accountants. Executives from Sosnik Bell could not be reached for comment.

A more lucrative place for victims of the fraud, however, are at the giant accounting firms that audited the investment managers who directed money into Mr. Madoff’s firm.

In several other fraud cases, accounting firms, which are responsible for scrutinizing the financial underpinnings of companies, have become targets for investor lawsuits. Ernst & Young paid $300 million to settle a lawsuit filed by Cendant related to fraud at one of the conglomerate’s subsidiaries. It had earlier paid $335 million to settle a lawsuit filed by Cendant shareholders.

Also last year, Pricewaterhouse agreed to pay $225 million to settle auditing malpractice claims tied to the Tyco scandal, which saw the convictions of top executives for grand larceny, conspiracy and securities fraud. Pricewaterhouse’s payment amounted to about 7 percent of total amount paid in Tyco lawsuits.

But the Madoff case presents an unusual situation, said Scott M. Berman, a partner at the law firm Friedman Kaplan Seiler & Adelman who represents investors in several feeder funds. Previous cases focused on the auditors of the firm at the center of the scandal, not the auditors of investment managers one rung removed.

“I expect that this is an issue that has not been litigated before,” Mr. Berman said.

With many of the feeder funds’ managers having taken losses from their own personal exposure to Mr. Madoff’s firm, the accounting firms may be a likely target for investors seeking to recoup at least some of their money.

PricewaterhouseCoopers was the main auditor for Sentry, the largest fund run by Fairfield Greenwich Group, the $14.1 billion investment manager that has lost the most money so far in the Madoff scandal. The accounting firm was tasked with minding Sentry, which had about $7.5 billion invested in Mr. Madoff’s firm.

“The company has not yet settled on a legal strategy,” said a Fairfield spokesman, Thomas Mulligan.

A spokesman for PricewaterhouseCoopers, Mike Davies, said, “No claim has been asserted against the PWC member firm in relation to Madoff, and we know of no valid basis for any claim.”

The lawsuit by New York Law School, filed in federal court in Manhattan last week, names J. Ezra Merkin, the money manager who placed $3 million of the school’s money into Mr. Madoff’s firm. But it also sues BDO Seidman, the American arm of BDO International and the auditor for one of Mr. Merkin’s funds, Ascot Partners.

In its lawsuit, New York Law School said that BDO Seidman had “utterly failed” in its auditing of Ascot Partners. The lawsuit says that BDO Seidman failed to flag Ascot’s reliance on a single money manager, Mr. Madoff, as well as Mr. Madoff’s reliance on Friehling & Horowitz.

BDO Seidman has said that it never audited Mr. Madoff’s firm, just Mr. Merkin’s, and that its audits of Ascot Partners “conformed to all professional standards.”

Mr. Berman, however, said the firm had a duty to dig deeper. “I don’t think that they can simply, blindly accept what Madoff did without doing their own auditing work,” he said.