Paul Konigsberg, an accountant for some of Bernard Madoff's biggest investors, pleaded guilty to conspiracy and falsifying records Tuesday under a plea deal that calls on him to cooperate with continuing government investigations.

Konigsberg, 78, of Greenwich, Connecticut, was the 15th person either convicted or to have pleaded guilty in a Madoff-related case, and the only remaining charged defendant whose case had not been resolved. His cooperation agreement indicates others may still face legal jeopardy.

In his plea, Konigsberg -- whose accounting firm had 300 clients with Madoff accounts -- admitted that he knew some investors were getting altered account statements from Madoff, and that he used the altered statements to file their taxes.

But he insisted he didn't know the whole business was a fraud.

"I was not aware of Madoff's horrific and evil Ponzi scheme, which has brought great suffering," said Konigsberg in a courtroom filled with rows of well-heeled family and friends.

He faces up to 30 years in prison and forfeiture of $4.4 million in addition to fines and restitution. But he could escape prison time altogether based on his cooperation.

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Prosecutors did not detail Konigsberg's cooperation. But in a release, they noted that he gave Madoff tax advice in 1995 on how to structure a taxable gift to two unnamed co-conspirators as a tax-free loan. The day before Madoff's arrest in 2008, they said, he called Konigsberg to discuss changing that 1995 transaction.

Madoff is serving a 150-year federal prison sentence.