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In a letter made public on Friday, federal prosecutors said that Madoff's former chief aide Frank DiPascali had provided "substantial assistance'' to investigators.
They said they anticipated writing an "extraordinary'' letter recommending leniency when he is sentenced for crimes including securities fraud and money laundering.
The memo to the judge was initially written in December but remained sealed until Friday.
Sections of the letter describing DiPascali's cooperation are still secret.
DiPascali pleaded guilty in August to helping Madoff implement his massive Ponzi scheme. He faces up to 125 years in prison.
Judge Richard Sullivan, in a previous hearing, DiPascali was a real risk of flight. Several past accused white collar criminals have fled in recent years including the CEO of Symbol Technologies, the one-time head of the Days Inn Hotel chain, the head of Comverse Technologies, among others.
DiPascali had said that before the Ponzi scheme unraveled he had always hoped Madoff would find a way to make enough money to repay investors.