BOSTON - His repeated warnings that Wall Street money manager Bernard Madoff was running a giant Ponzi scheme have cast Harry Markopolos as an unheeded prophet.

But people who know or worked with Markopolos say it wasn't prescience that helped him foresee the collapse of Madoff's alleged $50 billion fraud. Instead, they say diligence and a strong moral sense drove his quixotic, nine-year quest to alert regulators about Madoff.

"He followed through on everything he ever did. He never let up," said his mother, Georgia Markopolos, in an interview Thursday. "Some kids just let it go if it's too hard, but he wouldn't do that."

"He feels very sorry for these people that got taken," she added. "It wouldn't have happened if they would have listened to him long ago."

Markopolos waged a remarkable battle to uncover fraud at Madoff's operation, sounding the alarm back in 1999 and continuing with his warnings all through this decade. The government never acted, Madoff continued his ways, and people lost billions.

Markopolos reached his conclusion with the help of mathematicians like Dan diBartolomeo, whose analysis of the Madoff's methods in 1999 helped fuel Markopolos' suspicions.

"People should have seen the writing on the wall," diBartolomeo said.