MIAMI (Reuters) - Federal agents investigating a prominent Florida lawyer suspected of running an elaborate Ponzi scheme said yesterday the amount involved could exceed $1 billion, and they asked bilked investors to come forward.

At a news conference, investigators from the FBI and the US Internal Revenue Service (IRS) made a public appeal for information from individuals who had invested in a so-called Structured Settlement Investment scheme offered by 47-year-old Fort Lauderdale attorney Scott Rothstein.

Criminal charges have not yet been filed against Rothstein, who lived a luxury lifestyle and helped bankroll local politicians, campaign records show.

But a court document filed this week carried allegations that Rothstein masterminded a scheme in which he sold bogus or nonexistent legal settlements to unsuspecting investors since at least 2005. It said new investor money was used to pay previous investors in the classic Ponzi scheme model.

Monday's filing by the US Attorney for the Southern District of Florida was used to justify the seizure of properties, luxury vehicles and a yacht belonging to Rothstein.

Appealing for cheated investors to come forward with information, John Gillies, Special Agent in Charge of the FBI Miami Division, said the fraud investigation was going to be one of the largest ever seen in South Florida.

"I'll tell you, when it's all said and done, I estimate that this scheme could well exceed $1 billion," he said.

He added he did not think that Rothstein had acted alone: "I do not believe this was a one-man show."

He did not elaborate, and an FBI spokeswoman said the agency could not give details of an ongoing investigation.

"We will not be rushed; we will be thorough and we are far from over," Gillies added.

Rothstein, who returned from a trip to Morocco last week, has not been arrested and could not immediately be reached for comment. "He can run but he can't hide," Gillies said.