Accused swindler Bernard Madoff took the first public step to a guilty plea on criminal charges of running a $50 billion investment fraud over many years, according to court papers on Friday.

A document filed in U.S. District Court in Manhattan said U.S. prosecutors intend to file a criminal information in their case against Madoff upon the defendant's waiver of an indictment.

There was no indication when the information would be filed. Such a document is typically filed when a defendant is expected to plead guilty.

Prosecutors have until March 13 to indict Madoff or reach a plea agreement. Authorities said he confessed in December to running a giant Ponzi scheme, in which early investors are paid with the money of new clients.

This is the first step in order to enter a plea agreement, said Peter Henning, a professor at Wayne State University Law School in Detroit, and a former federal prosecutor. Most plea agreements are done by a criminal information rather than an indictment.

Madoff, a once-respected Wall Street trader and investment manager, was arrested and charged with one count of securities fraud on December 11. He is out on $10 million bail and under house arrest in his luxury New York apartment and 24-hour surveillance.

Madoff's lawyers were not immediately available to comment. The U.S. Attorney in New York declined comment.

(Reporting by Grant McCool and Martha Graybow; editing by Jeffrey Benkoe)