For more than two years, Herbalife Ltd. has been defending itself against allegations by a hedge-fund billionaire that the multilevel marketer (MLM) of nutritional products is a pyramid scheme. The tides may be turning in favor of Herbalife and against William Ackman, who has predicted Herbalife’s collapse and bet a fortune against the public company.

Federal prosecutors and FBI are investigating whether individuals including people hired by Ackman made false statements about the company in order to incite probes into Herbalife and lower its stock price, the Wall Street Journal reported Thursday, citing people familiar with the matter.

The Journal said investigators are reviewing public statements and allegations communicated to regulators by consultants and activists who have lobbied against Herbalife, as well as any connections or potential collaboration between those individuals and Ackman’s Pershing Square Capital Management. Neither Ackman nor Pershing Square had been served with a subpoena or had been visited by FBI agents in connection with the investigation, an anonymous source told the newspaper.

An FBI spokesperson declined to comment on the report, and the U.S. Attorney’s Office in the Southern District of New York didn’t respond Friday to a request for comment.

In a letter sent Friday to his investors, Ackman said Pershing Square is unaware “of any statements that we have made that are untrue, nor are we aware of any unlawful conduct on our part or by any consultants that we have hired."

For the last two years, Ackman has repeatedly called Herbalife a pyramid scheme and pressed regulators to investigate its business practices. FTC and the attorneys general in Illinois and New York are investigating the company, according to court records in a private lawsuit filed against Herbalife. Herbalife also has disclosed SEC requested information from the company.

Last year, FTC Chairwoman Edith Ramirez told a U.S. senator that since 1996 the agency has brought 15 cases against companies that were masquerading as MLMs and engaged in unlawful pyramid schemes.

Ackman first blasted Herbalife’s business model in a December 2012 presentation on Wall Street. He argued Herbalife is focused on rewarding distributors for recruiting others into its massive distribution network so they can buy products from the company and that few consumers outside the network actually buy Herbalife’s products.

Herbalife sells weight-management and nutrition products through a network of 4 million members in 91 countries.

The 35-year-old Herbalife has questioned its adversary’s motives and vigorously denied the pyramid scheme allegations.

The MLM, whose annual net sales last year totaled nearly USD $5 billion, discloses the average compensation of its distributors, which the company has said offers a great deal of transparency. Herbalife also has produced surveys that show most of its distributors—now called members—purchase weight-loss shakes and other products for their own consumption rather than as a business venture.

"Mr. Ackman has a $1-billion bet against Herbalife and a direct financial interest in hurting our company," Alan Hoffman, Herbalife’s executive vice president of global corporate affairs, said Thursday in a statement, commenting on the reported investigation by federal prosecutors into potential stock manipulation. “For more than two years, he has spent over $75 million orchestrating a false and fabricated attack against Herbalife, all in an effort to enrich himself. We are confident in the strong fundamentals of our business model and have remained committed to helping people and communities improve their nutrition, while knowing that one day his tactics would be exposed."

In its recent annual report filed with the SEC, Herbalife said its defense against Ackman’s claims includes “proactively reaching out to regulatory agencies and governmental authorities."