Investigators of Bernard Madoff's global swindle want more time to calculate the amount of money lost and the number of investors defrauded as they seek to determine restitution, but see no need to delay his June 29 sentencing, U.S. prosecutors said in court papers.

The documents, filed on Friday night, asked U.S. District Judge Denny Chin to delay a decision on restitution by three months, by which time prosecutors would provide an approximate amount for restitution.

Madoff, 71, pleaded guilty on March 12 to orchestrating Wall Street's biggest investment fraud of as much as $65 billion over about 20 years. He was immediately jailed and is expected to spend the rest of his life in prison.

The court-appointed trustee who is winding down Bernard L. Madoff Investment Securities LLC to return money to thousands of investors recently found microfilm records that may date back to 1979, according to the court papers. They need to be put in digital form and analyzed.

So far, the trustee has identified 1,341 account holders who collectively lost about $13 billion as of December 11, 2008, when the FBI arrested Madoff, the document said.

In its request to Chin, the government cited difficulties presented in the reconstruction of the record of defendant's fraud and victim loss in this case -- difficulties caused by the condition of the records kept by the defendant, as well as the scope and duration of the fraud.

In a fraud case, restitution is mandatory to those directly and proximately harmed the government said.

The requested order would permit the June 29 sentencing to proceed as scheduled, while allowing the effort to identify victims and losses to proceed apace, the court document said.

(Reporting by Grant McCool; Editing by Leslie Adler)