In letters Friday, the labor federation asked the Big Four accounting firms -- Ernst & Young, PricewaterhouseCoopers, KPMG and Deloitte & Touche -- to provide information on their potential involvement as outside auditors for companies now under federal investigation for possible rigging of option grants to boost their value to the recipients.

"Given the potential damage to shareholders due to options backdating, I am concerned about what role (name of accounting firm) may or may not have had in the backdating ...," the AFL-CIO's secretary-treasurer, Richard Trumka, said in the letters to the chief executives of the four firms, which were made public Monday. "I urge you to describe what steps are being taken to determine (name of firm)'s involvement in stock option backdating where it has occurred."

In backdating, options are issued retroactively to coincide with low points in a company's share price, a practice that can fatten profits for options recipients when they sell their shares at higher market prices. Backdating options can be legal as long as the practice is disclosed to investors and properly approved by the company's board. In some cases, however, the practice can break federal accounting and tax laws.

Spokesmen for PricewaterhouseCoopers and KPMG had no immediate comment on the AFL-CIO request. Ernst & Young and Deloitte & Touche spokesmen didn't immediately return telephone calls seeking comment.

Last week, government officials said they want to know what roles corporate directors as well as outside attorneys, accounting firms and compensation consultants might have played in helping executives manipulate the timing of option grants to enrich themselves and their colleagues.

More than 100 public companies, many of them in the technology sector, are under scrutiny by the Securities and Exchange Commission in the affair. The Justice Department is investigating scores of companies for possible criminal violations. And the Internal Revenue Service is looking at possible tax-law violations in option grants by some companies.

The potential cost to shareholders escalated Friday, when computer chip supplier Broadcom Corp. said it may need to boost a charge it takes to $1.5 billion or more for option accounting flaws -- double what it had estimated in July.

On Monday, chip maker Nvidia Corp. and software maker Wind River Systems Inc. both warned that they will miss regulatory deadlines for filing their most recent quarterly reports, joining a long list of tardy tech companies scrambling to clean up a stock options mess. The delay will expose both Nvidia and Wind River to being dropped from trading on the Nasdaq Stock Market. But that process takes several months, giving the companies time to comply with the SEC's reporting rules before getting bounced from the Nasdaq.

The AFL-CIO has some $400 billion in assets and is a major investor in companies, including many of those that are under investigation.