(Reuters) - A former manager at Bernard Madoff's firm was sentenced to six years in prison on Monday for helping her now imprisoned boss carry out his multibillion dollar Ponzi scheme.

JoAnn Crupi, 53, was the last of five former Madoff employees to be sentenced after a Manhattan federal jury found them guilty in March in the first criminal trial over Madoff's decades-long fraud, which collapsed in 2008.

U.S. District Judge Laura Taylor Swain also ordered Crupi, who worked in Madoff's investment advisory business for 25 years, to forfeit a symbolic $33.9 billion jointly with other defendants who worked at Bernard L. Madoff Investment Securities LLC.

Swain said at sentencing that Crupi served as the "reassuring voice of Madoff Securities," enabling the "devastating effects" of the crime.

"She was compliant with everything and questioned little," Swain said.

The five employees are among 15 people who have pleaded guilty or been convicted at trial. Madoff is serving a 150-year prison term after pleading guilty in 2009 to running a scheme that cost investors an estimated $17 billion or more in principal.

Prosecutors said the five employees knowingly propped up Madoff's fraud by creating fake documents and backdating trades.

Prosecutors said Crupi, who managed accounts purporting to have a $900 million balance in 2008, helped generate fictitious trading data presented to firm clients.

Prosecutors had sought more than 14 years in prison for Crupi, who was convicted of securities, conspiracy and other charges with the other four employees.

But Swain imposed a more lenient term, as she did in sentencing the other employees, a trend a prosecutor last week said set a bad precedent.

Daniel Bonventre, Madoff's former back office director, was sentenced last Monday to the longest prison term, 10 years.

Annette Bongiorno, Madoff's former secretary and later a manager, received six years in prison, while computer programmers Jerome O'Hara and George Perez received 2-1/2 years in prison each.

Prosecutors sought more than 20 years for Bonventre and Bongiorno and more than eight for Perez and O'Hara.

The defendants, who are expected to appeal, have said Madoff deceived them into believing the business was legitimate.

Crupi told Swain on Monday that she had believed Madoff, who had discouraged her from learning about the securities industry.

"Knowing my work played a part in carrying out this horrible scheme will cause me shame and remorse for the rest of my life," Crupi said.