Facing the threat of criminal indictment and a torrent of civil lawsuits over its marketing of abusive tax shelters, KPMG LLP is fighting back -- against the very customers to whom it sold the deals.

Lawyers for KPMG recently filed court papers in connection with a Texas case signaling that they will seek to ask plaintiffs and their personal tax advisers what they knew about the shelters and why they failed to disclose them on their individual income tax returns.

The accounting firm contends that many former clients bear responsibility for using tax-avoidance deals with esoteric names such as BLIPS, OPIS and FLIP -- and that they should not enjoy a free ride to recover millions of dollars from KPMG.

At the same time, KPMG is sending out letters to plaintiffs in other cases asking what they expected to gain from the tax shelters and what they told the Internal Revenue Service about them.

More than half the lawsuits filed against KPMG involve instances in which taxpayers may have misled authorities about the purpose of the deals or failed to report them in tax filings, according to a source close to the situation who spoke on the condition of anonymity because the cases are in their early stages.

The hard-nosed strategy comes just weeks after the New York firm acknowledged "full responsibility for the unlawful conduct by former KPMG partners." The U.S. attorney in New York continues to investigate tax services KPMG offered to companies and wealthy individuals between 1996 and 2002, as well as the firm's alleged refusal to turn over documents to federal authorities. Tax shelters sold by KPMG may have helped its clients avoid paying about $1.4 billion in taxes, federal lawmakers said.

While it is pushing back hard against civil plaintiffs, the firm is negotiating with prosecutors to try to head off an indictment -- a step that hastened the death of Arthur Andersen LLP in 2002, when it was charged with obstructing justice for work connected to client Enron Corp.

KPMG has pushed out its former deputy chairman and the head of the tax services office, as well as the former partner in charge of the personal financial planning unit. More than two dozen individuals with ties to the tax shelters remain under investigation, according to defense lawyers with ties to the case who spoke on the condition of anonymity because of the sensitivity of the negotiations.

The probe began with hearings by the Senate permanent subcommittee on investigations in November 2003. The panel concluded that KPMG improperly pressured employees to sell tax shelters, essentially "turning them into salespersons." In some cases, KPMG continued to peddle the structures even after IRS officials deemed them invalid. Agency officials also have accused the firm of withholding key documents about the tax shelters.

KPMG's tax unit generated more than $1.2 billion in annual revenue and has more than 100 offices across the country, according to a Senate report filed in April.

Lawyer Melvyn I. Weiss, who has filed another shelter-related case against KPMG, said the accounting firm's stance is an effort to convince the courts that sophisticated clients knew what they were getting into when they purchased the shelters.

Paul J. Dobrowski, a Houston lawyer representing plaintiffs in the Texas case, declined to comment. Gerald H. Silk, a plaintiffs' lawyer who has filed a lawsuit against KPMG in Arkansas, said the firm has been pursuing aggressive defense tactics.

"Look, there's substantial liability here," said Silk, of the New York law firm Bernstein Litowitz Berger & Grossmann.