By MITCH WEISS In this Feb. 28, 2013 photo, Sarah Chavez, center, sits with her son Bidal, right, and daughter Sarahi, front, at her home in Lexington, N.C. Desperate to raise money for their 6-year-old daughter’s cancer treatments last summer, friends told Jose and Sarah Chavez of a way to quickly turn their meager savings into a small fortune. But what the Chavez family and many others didn’t know was that state and federal regulators for months had received complaints that ZeekRewards was a scam. (AP Photo/Chuck Burton) LEXINGTON, N.C. (AP) — People are questioning how a North Carolina man operated a $600 million Ponzi scheme that attracted 1 million investors, even though regulators had received dozens of complaints about the company. Authorities say Paul Burks is the mastermind of one of the biggest investment scams in U.S. history. His online company ZeekRewards was based in Lexington, N.C. The Securities and Exchange Commission closed ZeekRewards in August. Burks has agreed to pay a $4 million penalty. Investigators say he siphoned millions for his personal use, but he hasn’t been charged. Burks says he never told anyone to invest more money than they could afford. Investors say they were duped. They also didn’t know regulators had received nearly a dozen complaints about ZeekRewards but failed to take action for months.