“There was a tremendous amount of guilt being there,” watching Mr. Cook lure investors, said Mr. Schlobohm in an interview, the first in which he has spoken publicly about how he helped put Mr. Cook behind bars. “Knowing this was a fraud with the highest degree of certitude, and having to watch people in the process of losing their life savings, was extremely difficult.”

The United States attorney for Minnesota prosecuted the case against Mr. Cook, and the Commodity Futures Trading Commission and the Securities and Exchange Commission are both pursuing civil suits against Mr. Cook and helped with the federal investigation. Those agencies point to the Trevor Cook case as an example of the positive lessons authorities learned from the Bernard L. Madoff scandal and other regulatory debacles. (In the Madoff case, tipsters warned regulators for years of problems, but they did not take action until Mr. Madoff’s fund collapsed.)

For all of the Justice Department’s efforts, though, only about 5 percent of the $160 million invested in Mr. Cook’s scheme has been recovered.

And for his part, Mr. Schlobohm says that his time as a whistle-blower was often an ordeal, leaving him worried about his safety and that of his family. After he began acting as an informant, he was spending considerable time on the case. He and his employer, an investment company, agreed that it was better for him to quit than to risk dragging the firm into the probe.

“There were definitely times when I was fearful,” he says. “I had to ask questions and dance on a tightrope, pretending I was going to help them bring money in without being obvious I was working with the federal regulators. They certainly knew what they were doing was a fraud, so I was surprised that they weren’t thinking that maybe someone else had discovered it.”

ONE day in early 2009, Ty Schlobohm was visiting the 117-year-old Romanesque mansion that Mr. Cook had bought for $2.8 million and outfitted as a modern trading floor, where walls of flat screens flashed currency prices from around the globe.

“I was walking down the stairs, and on the first floor they had a big conference room,” Mr. Schlobohm recalls. “There sat no less than 25 women, all north of 65 to 70 years old, at an investment seminar where one of the salesmen was waxing poetic about the strategy. I was fairly certain at that point that I was looking at some degree of fraud.”