NEW YORK: One day after Wall Street huckster Bernard Madoff was sentenced to 150 years in prison, investigators remain stumped about the whereabouts of the billions of dollars he stole from his victims.

Federal Judge Denny Chin, at a sentencing hearing Monday, handed the 71-year-old Madoff the toughest possible sentence, placing him behind bars for the rest of his life.

Chin also gave federal authorities an additional 90 days they requested to locate a mountain of money swindled by Madoff, the former Nasdaq chairman once among the most respected figures in the investment world.

Madoff was the mastermind behind one of the world's biggest financial scams, which unraveled during the global economic meltdown late last year, robbing thousands of investors of retirement nest eggs and college funds.

Among his victims were Hollywood stars, international celebrities, several of the world's best-known banks and various Jewish charities, some of which were forced to close after the scheme collapsed.

The fraudster meanwhile enjoyed all the trappings of a fabulously wealthy lifestyle, including four yachts, a Manhattan penthouse, a Florida mansion, a pied-a-terre in the south of France and other luxuries.

Prosecutors have found that some 13 billion dollars were invested with Madoff by his clients, but a court trustee has only been able so far to recover just 1.2 billion.

Deceived clients meanwhile had believed their money was actually invested and by now should have been worth some 65 billion dollars.

With most of the money missing and unaccounted for, questions are swirling during the investigation: Who pocketed the rest of the money -- a sum exceeding the gross national products of numerous countries?

How likely is it that the victims will ever receive restitution? And what responsibility do authorities have to restore the victims' lost assets?

During Monday's sentencing hearing, one weeping victim pleaded with Madoff to divulge whether he might have deposited the stolen cash in an overseas bank.

Madoff's lawyer Ira Sorkin maintained at the sentencing hearing that no funds had been squirreled away in foreign accounts.

"Mr Madoff did not hide money overseas," the attorney said.

Financial expert Roger Siefert told AFP that there are two likely scenarios for where the money went.

In the first one, the funds would have gone "into the pockets of the Madoff family for personal use or to run the business," said Siefert, a managing director in the LECG financial consultancy in New York.

"In that case, significant amounts of it should be traceable given enough time and energy," he said.

Madoff meanwhile has said that he acted alone and that his relatives have no knowledge of the massive investment scam.

Siefert said a second theory relates back to the very nature of a pyramid scheme -- much of the money that flowed into Madoff's investment venture over the years probably was paid out to early investors who cashed in in what they believed to be a legitimate investment venture.

"In my opinion, this was the primary use of the funds," Siefert said.

"Madoff was promising yields of approximately 15 percent year-over-year. An early investor would not have had to leave his or her money in a Madoff fund for long to have experienced significant growth in their investment," he said.

Indeed, Madoff attorney Sorkin suggested Monday that that is precisely what happened, explaining that payouts were made from the fraudulent fund over the years to investors.

"There was money in and money out," Sorkin said.

For many of the victims, the question now no longer is where the money went, but whether the government will make them whole after failing year after year to detect the massive fraud scheme.

Michael De Vita, 59, a victim and the founder of an association of about 100 victims "Madoff's Survivors," laid some responsibility for the scandal on the shoulders of the Security and Exchange Commission (SEC) and federal authorities tasked with overseeing investment institutions, which failed miserably in Madoff's case despite several warnings.

While he had tough words of condemnation for Madoff, De Vita was almost as hard on the SEC, which he described as "incompetent, corrupt or criminally negligent" in its oversight of the finance sector.