Washington ? WorldCom Inc., already facing fraud charges involving almost $4 billion in disguised expenses, told the government Monday it is investigating possible new accounting problems with its reserve accounts.

The Nasdaq Stock Market said it would remove shares of the embattled telecommunications company from trading on Friday. Investors pummeled WorldCom stock, which plunged 90 percent to 6 cents a share in Monday’s early trading after a three-day halt that followed its disclosure of accounting irregularities.

More than 250 million shares changed hands in frenzied trading in the first 15 minutes. By noon, WorldCom, which once topped $64 a share, had become the most heavily traded stock ever in a single day in U.S. history and was valued between 6 cents and 7 cents. During the day, 1.47 billion shares of WorldCom changed hands, at prices as high as 15 cents and as low as 5 1/2 cents.

Before the market opened, the company submitted a sworn statement to the Securities and Exchange Commission, which had demanded a report after filing civil fraud charges.

The SEC is continuing its civil investigation of the company, and SEC Chairman Harvey Pitt said on television: “Criminal charges may be too good for the people who brought about this mess.”

WorldCom’s woes got deeper.

The company, which already had laid off thousands of employees, said it had defaulted on $4.25 billion in bank loans.

Shareholders sued WorldCom in federal court in Mississippi, where it is based. The class-action lawsuit said shareholders paid artificially inflated prices for the stock because the company had failed to disclose significant adverse information in its financial reports.

In its statement to the SEC, WorldCom said its audit committee was reviewing records for 1999 through 2001 because questions were raised about significant changes in reserves against potential financial losses.