U.S. investigators say 10 or more people associated with imprisoned swindler Bernard Madoff could face criminal charges in the coming months, a law enforcement source said on Tuesday.

The source, who asked not be identified because of the continuing investigation into the multibillion-dollar Madoff fraud, said the FBI was closer to the beginning than the end of the probe.

Disgraced financier Madoff, 71, was handed a 150-year prison sentence Monday. He was arrested in December and pleaded guilty in March to orchestrating Wall Street's biggest investment fraud of as much as $65 billion.

He has not named accomplices in his classic cash in, cash out Ponzi scheme. The only other person charged criminally so far is his outside accountant, David Friehling.

There will probably be more people charged, the law enforcement source said. It is likely to be 10 or more, but it is going to be a lengthy process that could take months or more.

Madoff was prosecuted by the Office of the U.S. Attorney in Manhattan, which declined comment on its continuing investigation.

In pre-sentencing court papers, prosecutors said Madoff organized and led the fraud and had numerous clerical employees working in the business.

It would not have been possible to execute his scheme without their assistance, prosecutors said.

Federal investigators have declined to identify who is the focus of their inquiries, but they are skeptical of claims by some people who worked at the Madoff firm that they had no knowledge of a fraud spanning decades, the law enforcement source said.

JUGGERNAUT

White-collar crime experts have said from the start that Madoff's scheme appeared to be too complex to carry out alone.

What will happen from now is the juggernaut of the federal government and many other agencies are going to roll forward and scoop up as many people as they can and prosecute them, said Anthony Sabino, professor of law and business at St. John's University in New York.

Bernard L. Madoff Investment Securities LLC had about 200 employees in a brokerage unit and an investment advisory business. The court-appointed trustee winding down the firm said the criminal activity took place in the investment arm.

Madoff's wife of 45 years, Ruth, their sons Mark and Andrew and his brother Peter and niece, Shana, all face civil lawsuits. Peter, Mark and Andrew Madoff were executives in the brokerage, but their lawyers have said they were not aware of the fraud.

Other long-time employees identified in lawsuits or other court documents include former financial manager Frank DiPascali, a 33-year veteran, JoAnn Crupi and Annette Bongiorno, who worked there for 25 years and 40 years, respectively.

Their lawyers have not returned calls seeking comment.

As part of trustee Irving Picard's job to recover as much money and assets as possible to redistribute to thousands of swindled investors, he has sued several Madoff insiders -- billionaire businessmen accused of acting as feeders.

The trustee and regulators say these fund managers knew or should have known the financier was running a fraud.

They include $3.5 billion sought from Fairfield Greenwich Group founded by Walter Noel and Jeffrey Tucker. Massachusetts regulators have also sued the group on civil charges.

In response, the fund has said the group and its employees were also victims of the Madoff fraud.

The U.S. Securities and Exchange Commission and the trustee last week sued Cohmad Securities, a brokerage founded by Bernard Madoff and long-time associate Maurice Cohn. Also sued were Cohn's daughter, Marcia, and Robert Jaffe, both Cohmad executives.

A total of about $6.5 billion is being sought by Picard from hedge fund founder Ezra Merkin and businessmen Stanley Chais and Jeffry Picower. New York Attorney General Andrew Cuomo also has sued Merkin.

(Reporting by Grant McCool; editing by Andre Grenon)