Bernard Madoff is set to admit he orchestrated Wall Street's largest fraud on Thursday, but there are still big questions about the case, including whether anyone else will face criminal charges.

The once-respected money manager, accused of stealing billions of dollars from clients of his family-run business, is the only person who has been charged by federal prosecutors.

Legal experts who have been following the case, however, believe the purported scheme was likely too complex and went on too long to have been carried out by one person alone.

A key issue is whether the conduct of anyone else associated with Madoff rose to the level of criminal activity, experts say. Prosecutors have said their investigation is continuing.

Prosecutors said in court papers on Tuesday that Madoff hired people with little experience in the securities industry and directed them to concoct phony trading records and account statements. These employees were not named and there was no indication whether any charges would be brought against them.

We know that others were involved who helped him, said Christopher Steskal, a former federal prosecutor who is in private practice at law firm Fenwick & West in San Francisco. What is criminal is if people have knowledge that a fraud is occurring.

Madoff employed family members including his brother, Peter, and his two sons, Mark and Andrew, at his firm, which operated a brokerage business and a money management unit.

His brother and sons have said previously through their lawyers they were not aware of any fraud. Authorities have not accused them of any wrongdoing.

CONFESSED TO SONS

Madoff was arrested three months ago after his sons, who worked for the brokerage side of the business, told their lawyer he had confessed his massive Ponzi scheme to them. The lawyer then called authorities.

There is nothing in the charges to suggest that Mark and Andrew Madoff had any knowledge of the fraud or that market making and proprietary trading were anything other than legitimate businesses, Martin Flumenbaum, their attorney, said in a statement on Wednesday.

Peter Madoff worked as the overall firm's compliance officer.

Any suggestion that Peter Madoff knew that his brother was engaged in this Ponzi scheme is absurd, his attorney said in a statement.

There are also questions about Bernard Madoff's wife, Ruth, who has asserted through her lawyer in court papers that about $70 million in her name is unrelated to the purported fraud and should be hers to keep. She has not been accused of any wrongdoing. But investors' lawyers have said they believe any assets she holds should be subject to seizure.

Other questions about the case include what happened to all of the money deposited by investors and whether they will be able to get any of it back, as well as the precise amount of their collective losses.

It may be impossible to calculate that figure because the scheme ran for so long and involved so many investors, experts say.

There is likely a lack of records going back from the inception of the scheme, said George Jackson, an attorney in the Chicago office of law firm Bryan Cave and a former federal prosecutor.

NEW ESTIMATE

U.S. prosecutors have given a new estimate of the size of the worldwide scheme -- $64.8 billion -- up from the $50 billion fraud they said Madoff confessed to in December.

The government said in court papers that the $64.8 billion was based on the purported amount held collectively by Madoff investors as of November 30, 2008, based on about 4,800 client accounts.

In reality, prosecutors said, the Madoff firm held only a small fraction of that balance.

A trustee overseeing the liquidation of Madoff's brokerage said last month he had recovered less than $1 billion for customers -- a fraction of their overall deposits. Prosecutors are separately demanding fines, forfeiture and restitution from Madoff, but it is unclear how much money he has.

Madoff is expected to plead guilty to all 11 counts filed against him when he appears in court on Thursday, his lawyer told a judge on Tuesday.

Prosecutors say there is no plea agreement with Madoff -- a type of deal in which defendants agree to enter a guilty plea and cooperate with the government in exchange for the possibility of more lenient treatment at sentencing.

His charges could bring 150 years in prison under sentencing guidelines.

Madoff has been under house arrest at his Manhattan penthouse apartment, but prosecutors may seek to jail him until his sentencing in several months' time.

Former prosecutor Steskal said it is unclear why Madoff appears set to plead guilty without a plea deal. He said prosecutors may have refused to offer one, but if that were the case then Madoff could have contested the charges.

It's hard to know what is motivating him, Steskal said. It may be as simple as he simply wants it over with.

The case is USA v Madoff 09-213 in U.S. District Court for the Southern District of New York (Manhattan)

(Editing by Brian Moss and Andre Grenon)