NEW YORK - For two years, the two sons of jailed financier Bernard Madoff portrayed themselves as whistle-blowers who exposed their father's huge fraud.

The suicide of Mark Madoff could leave unanswered questions for investors seeking restitution for the billions of dollars his father siphoned off - and for criminal investigators who have continued to pursue Madoff's family.

Mark Madoff, 46 - Bernard Madoff's older son - hanged himself Saturday with a dog leash from a metal ceiling beam in his Manhattan loft, as his 2-year-old son slept in another room. The death was officially ruled a suicide by hanging Sunday by the city medical examiner.

Mark Madoff died on the anniversary of his father's arrest two years ago in the largest Ponzi scheme ever recorded.

It followed the filing in recent weeks of dozens of lawsuits by court-appointed trustee Irving Picard as he pursues billions of dollars in damages against those who profited from the fraud.

Increasingly, Picard has used sharp language in lawsuits against those who knew Bernard Madoff well. On Wednesday, he included the brothers as defendants in a lawsuit he brought against the London-based international arm of Madoff's business, saying the overseas operation was used to siphon money from the fraud for the family.

Mark Madoff had worked with his father at the company since 1986. His brother, Andrew, had been there since 1988.

Investors on Sunday said Mark Madoff's death would make it harder to find answers to how many people knew of the fraud.

But Ruth Goldstein, wife of investor Allan Goldstein, 78, of Great Barrington, Mass., who lost his life savings in the fraud, called the suicide "the ultimate punishment" for Bernard Madoff.

Madoff is serving a 150-year prison sentence in a North Carolina prison. He admitted having told his clients in November 2008 that $20 billion in investments had grown to $65 billion when only several hundred million dollars remained.

Picard sought $200 million from Madoff family members in his lawsuit, which charged that the family used the Madoff business like a "piggy bank," spending tens of millions on lavish homes and other luxuries.

Mark and Andrew Madoff portrayed themselves as heroes who sought to stop their father from committing more fraud.

On Sunday, Martin Flumenbaum, their lawyer, continued to portray them as innocent. "Mark and Andrew Madoff had no prior knowledge of their father's crimes," he said.

A criminal investigation is also active against the family. At a minimum, authorities believe they might be able to build a tax-fraud case, alleging that Madoff family members failed to fully report all of the income they received directly and indirectly from the family business.

But law enforcement officials said Sunday that they had had no direct contact with either of the brothers in the last year, and said no charges were imminent.