With Bernard Madoff behind bars, investigators have their work cut out for them identifying who else may have been involved in his nearly $65 billion scam.

Among other things, prosecutors must determine what role, if any, Madoff's wife, brother, two sons and employees played in perhaps the largest Ponzi scheme in history.

"A lot of resources and effort are being expended, both to find assets and to find anyone else who may be responsible for this fraud," federal prosecutor Marc Litt said at a hearing Thursday where Madoff pleaded guilty to 11 felony counts and was ordered to jail immediately.

Authorities also will have to reconcile Madoff's statement to the judge that "to the best of my recollection, my fraud began in the early 1990s." Prosecutors have alleged that the swindle began in the 1980s.

Even though he admitted that his investment advisory business was crooked, Madoff insisted to U.S. District Judge Denny Chin that the other businesses his firm engaged in — those run by his brother and two sons — "were legitimate, profitable and successful in all respects."

After Madoff's statement, Assistant U.S. Attorney Marc Litt told Chin the government "does not entirely agree" with Madoff's description of the crime and went on to say, "At times, his firm would have been unable to operate but for the cash generated from this Ponzi scheme."

Madoff's wife, Ruth; brother, Peter; and sons, Mark and Andrew; have not been accused of any wrongdoing. But unhappy investors who lost their fortunes with Madoff believe others were involved.