After 15 convictions, federal case against Bernard Madoff could be over
Bernard Madoff fraud victims holds a news conference and protest following the sentencing hearing for Madoff on June 29, 2009 in New York City [AFP]

After more than five years and 15 convictions, the U.S. government's criminal investigation of Bernard Madoff's colossal Ponzi scheme may finally be coming to an end.

In court filings on Tuesday and Wednesday, prosecutors in New York asked judges to schedule sentencing dates for key witnesses who pleaded guilty and agreed to help investigators, including Madoff's right-hand man, Frank DiPascali.

Cooperators are typically sentenced only after an investigation is complete, in order to give judges a full picture of the extent of their assistance.

The filings suggest that prosecutors do not expect to charge anyone else.

Bernard Madoff, 76, is serving a 150-year prison sentence after pleading guilty in 2009 to masterminding the scheme, which cost investors an estimated $17 billion in principal.

Last year, Reuters reported that prosecutors could still be building a case against Madoff's son, Andrew.

However, he died in September after a long battle with cancer. Bernard Madoff's other son, Mark, committed suicide in 2010 on the second anniversary of his father's arrest.

Both sons denied knowing about the fraud. They turned their father in to authorities in December 2008.

DiPascali, who pleaded guilty in 2009, was the star government witness in the trial of five former Madoff employees, who were found guilty last March of helping Madoff cover up his fraud for decades.

Other cooperators to be sentenced include David Friehling, Madoff's accountant; Enrica Cotellessa-Pitz, the controller at Madoff's firm; and David Kugel, a trader who backdated trades to prop up the scheme. All testified at the trial of the former Madoff employees.

On Wednesday, U.S. District Judge Laura Taylor Swain, who oversaw the trial, set May sentencing dates for Cotellessa-Pitz, Kugel and two other cooperators.

DiPascali and Friehling, whose cases are before other judges, do not yet have sentencing dates. It remains possible both will be transferred to Swain for sentencing.

Swain sentenced the five Madoff workers - back-office director Daniel Bonventre, portfolio managers Annette Bongiorno and JoAnn Crupi and computer programmers Jerome O'Hara and George Perez - to between 2-1/2 and 10 years, drawing criticism from prosecutors that their punishment was too light.

In a rare step, the office of Manhattan U.S. Attorney Preet Bharara said it would appeal those sentences.

(Reporting by Joseph Ax; Editing by Noeleen Walder and Ken Wills)