Prosecutors made the revelation as they told a judge in court papers that there was no need to order restitution because all of Madoff's assets will be distributed to investors through forfeiture requirements.
As part of their filing in U.S. District Court in Manhattan, they summarized the findings of a court-appointed trustee who is calculating how much investors lost so it can be decided how to divide up assets that are recovered.
The government said a search of financial records, including microfilm records dating back to 1979, show that investors suffered net losses exceeding $13 billion. In all, 15,870 claims have been made to the trustee by those seeking a share of any recovered money.
The court document said there were approximately 8,094 customer accounts held by Madoff's private investment business from at least 2000 through December 2008. At the time of his Dec. 11 arrest, there were 4,902 active customer accounts, the government said.
Most of the customers who had current accounts have made claims with the trustee, prosecutors wrote. Of those, nearly 50 percent had a net loss, meaning they contributed more funds to their accounts than they withdrew, while about half had no net loss because they withdrew more funds than they contributed, prosecutors said.
The court filing said that about 83 of the active customer accounts were held by investment funds that put their clients' money with Madoff.
Authorities have identified more than $1 billion in assets that can be distributed to victims.
Madoff, 71, was sentenced to 150 years in prison after he oversaw the manufacturing of fake financial statements to support his claims that investor accounts had swelled in value to $65 billion last November when there actually was only several hundred million dollars left.
His multi-decade fraud destroyed thousands of people's life savings, wrecked charities and shook confidence in the financial system. He carried it out even as he seemed among Wall Street's elite. He served for a time as a Nasdaq chairman and survived scrutiny several times from the Securities and Exchange Commission.
At his sentencing, prosecutors secured a $171 billion forfeiture order which mirrors the amount they estimate that flowed in and out of Madoff's investment business over the decades.