NEW YORK (AP) - A judge in New York challenged the government to prove that jailed financier Bernard Madoff's former finance chief is a valuable co-operator on Wednesday, implying he may only be pretending to help prosecutors in the hopes of garnering a lighter sentence.

US District Judge Richard Sullivan turned down a bail request for Frank DiPascali a second time, rejecting a $10 million bail package as insufficient to ensure he does not flee.

DiPascali, 52, pleaded guilty in August to helping Madoff's multi-decade Ponzi scheme that cost thousands of investors billions of dollars. Madoff is serving a 150-year prison sentence.

Sullivan challenged Assistant US Attorney Marc Litt to prove that DiPascali's cooperation was valuable in the search for truth in a multidecade fraud that financial regulators say was kept secret for so long in large part because of DiPascali's shrewd financial maneuvers.

He said Litt could submit a secret document that would show how DiPascali's cooperation was aiding the prosecution of others. Litt said he will consider making such a submission.

DiPascali's lawyer, Marc Mukasey, suggested that his client was providing evidence that will be used to arrest others in the case, saying he was eager for Sullivan and other federal judges to hear his client prove his co-operation by telling his story on the witness stand.

Sullivan, though, was reluctant to endorse DiPascali's co-operation based on the claims of government prosecutors and Mukasey, himself a federal prosecutor in Manhattan until several years ago.

"The parties are basically telling me I should trust them and leave it to the professionals," the judge said.

Yet, he added, the government did not seem to have "the slightest clue about what turned out to be the biggest fraud in US history" before Madoff gave himself up in a confession to his family and the FBI last December.

Sullivan said DiPascali might be continuing to act fraudulently in "a last stand" after a decade of lies by pretending to co-operate in the hopes of reducing a potential maximum sentence of 125 years in prison.

As Sullivan spoke, the bespectacled DiPascali dropped his head into his hands. At other times, he shook his head.

Still, Mukasey said his client was even more enthusiastic to help the government after Sullivan ordered him jailed without bail to the surprise of prosecutors at an earlier bail hearing.

Sullivan called DiPascali's role in the fraud "crucial" and said his potential sentence was "astronomical", fueling an incentive to flee that might be greater than safeguards that would include electronic monitoring and $2 million in cash or property to be posted by others.

At one point, the judge even seemed to taunt investigators, saying DiPascali's co-operation might already be devalued because two people he might be cooperating against "are in prison in Butner, North Carolina, or at the bottom of a swimming pool."

Madoff, 71, is housed at the Butner Federal Correctional Complex near Raleigh, North Carolina. On Sunday, Jeffry Picower, 67, was found dead at the bottom of a pool at his oceanside mansion in West Palm Beach, Florida, just weeks after authorities accused him in a civil action of receiving more than $7 billion in money from Madoff that he should have known was the result of fraud.

Authorities have said Picower, who had maintained he did not know about the epic fraud and was a victim himself, died of an apparent heart attack.