BOSTON (Reuters) - "America's Top Ranked Money Manager," read the 2003 headline in The Wall Street Digest, an investment newsletter.

Fast-forward six years: that money manager, Arthur Nadel, has vanished along with an estimated $350 million of his clients' money, and investors are fuming over the glowing report promoting the "unusual success" of the Florida hedge fund manager, who disappeared last Wednesday.

As a probe widens into Nadel, his burned investors are seeking answers. Some say they were blindsided by the losses. Others are quoted as saying they saw warning signs recently.

Some, like 68-year-old Tony Hagar, say they were drawn to his funds by The Wall Street Digest and the upbeat report by its editor, Donald Rowe, and now question how much due diligence the investment newsletter industry conducts.

"He seemed to indicate that they were a reasonable investment. They put out a letter that says these folks have done substantially well, and that Don Rowe had looked at them closely," said Hagar.

Hagar lost $1.5 million, nearly his entire retirement savings, along with any hope of retiring next year as planned from the Embry-Riddle Aeronautical University, where he works as a professor in Daytona Beach, Florida.

"That's not going to happen," he said.

The FBI's Tampa office opened an investigation into Nadel on Tuesday, launching a search for the 76-year-old former New York jazz pianist whose disappearance has drawn parallels to last month's arrest of former Nasdaq Stock Market chairman Bernard Madoff.