NEW YORK (CNNMoney.com) - Federal and state authorities are investigating Web sites that promise to generate generous returns to viewers who look at their ads, according to a report published Friday.

These so-called "auto surf" sites are Internet versions of a classic Ponzi scheme, a type of fraud that promises vast returns to investors but pays them with money from subsequent investors instead of revenue generated by business, the Wall Street Journal said.

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One of the largest such sites, 12dailyPro, attracts visitors by offering them free memberships. But in order to earn cash, members have to "upgrade" their membership in $6 increments, with a maximum investment at any one time of $6,000, the newspaper said.

These sites, which piggyback on the surge in Internet advertising, have drawn scrutiny as law enforcement authorities focus more on Internet crimes, the report said.

The 12dailyPro site, which recently claimed it had 300,000 members from around the world, is being investigated by the FBI, the Securities and Exchange Commission and at least two states, the newspaper said, citing people familiar with the matter.

The FBI's Internet Crimes Complaint Center received 207,000 complaints in 2004, the latest numbers available, up 66 percent from the previous year, but the agency says only a small percentage of those were related to investment fraud, according to the newspaper.

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