If the Rigases didn't disclose the alleged payments as income on their tax returns, they could face felony charges of tax evasion or filing false returns. Tax evasion carries a penalty of up to five years' imprisonment per count. The penalty for filing false returns is up to three years' imprisonment per count. Any such charges would likely hinge on a conviction or guilty plea in the original fraud case because prosecutors would need to show that the income was illegally obtained.

The existence of the tax investigation signals that the government is throwing all of its legal ammunition at the case. After the Rigases were routed last month from their company-owned apartment on New York's Upper East Side in a dramatic early-morning arrest, President Bush hailed the case as the latest effort by the government to crack down on corporate crime. As the government seeks to restore confidence in stock markets shaken by corporate scandals, it is going out of its way to punish alleged wrongdoers publicly.

Although it would be unusual for charges to be filed against the same people in separate jurisdictions, government officials decided that prosecutors in Pennsylvania would focus on tax issues because they wanted a piece of the broad investigation of Adelphia, which is based in Coudersport, Pa. Prosecutors are still considering bringing criminal charges against the company, which was accused of fraud last month in a civil complaint filed by the Securities and Exchange Commission.

Mr. Rigas, the company's 78-year-old founder and former chairman, and sons Timothy J. Rigas and Michael J. Rigas, the former chief financial officer and former operations vice president, respectively, were charged last month with securities fraud, wire fraud and bank fraud. Adelphia has said it supports the arrests.

In one startling allegation, the elder Mr. Rigas is accused of racking up more than $66 million in personal debt and then withdrawing so much money from the company for personal use that his son Timothy had to limit him to $1 million a month. The elder Mr. Rigas's annual compensation was listed at $1.9 million in public filings.

The criminal complaint also alleges that the Rigases used the company's three corporate jets for personal use that was neither approved by the board nor paid for by the family. Adelphia also allegedly paid for two Manhattan apartments, one used rent-free by John's daughter and son-in-law, who was a member of the board at the time.

Two other former Adelphia executives -- James R. Brown and Michael C. Mulcahey -- were also charged last month. The tax probe, however, is focusing on the Rigases.

Peter Fleming, a lawyer for the elder Mr. Rigas, said he was unaware of the tax investigation and declined to comment. A lawyer for Timothy Rigas, who has previously said his client would "defend the charges vigorously," declined to comment on the tax probe. A lawyer for Michael Rigas didn't return phone calls. A spokesman for the U.S. Attorney's Office for the Middle District of Pennsylvania declined to comment.

Under federal law, prosecutors in New York have until Sept. 3 to move their case against the Rigases forward with an indictment. If that doesn't happen, the Rigases would be granted a hearing at which evidence against them would be presented. In many cases, plea negotiations are conducted before the deadline for an indictment is reached.

Although attorneys for the Rigases approached prosecutors before the charges were filed to offer the family's version of events, another person familiar with the matter said there has been virtually no contact since then and that the family members are considering taking the case to trial.

The possibility of new charges involving tax evasion represent "another battlefield for the defense to worry about," said Robert Mintz, a former federal prosecutor in New Jersey who now heads the white-collar-crime practice group at McCarter & English in Newark, N.J., and who isn't involved in the case.

"If the Rigases were simply drawing money out of the company and concealing the fact that they were receiving payments, that's a very straightforward case for tax evasion and under-reporting your income," he said. "It's like getting a bonus and not reporting it as income."