LEXINGTON, North Carolina (AP) - People are questioning how a North Carolina man operated a US$600 million (S$744 million) fraudulent investment scheme that attracted a million investors, even though regulators had received dozens of complaints about the company.

The authorities say Paul Burks is the mastermind of one of the biggest investment scams in United States history. His online company ZeekRewards was based in Lexington, North Carolina.

The Securities and Exchange Commission closed ZeekRewards in August. Burks has agreed to pay a US$4 million penalty.

Investigators say he siphoned millions for his personal use, but he has not been charged. Burks says he never told anyone to invest more money than they could afford.

Investors say they were duped. They also did not know regulators had received nearly a dozen complaints about ZeekRewards but failed to take action for months.