The Federal Bureau of Investigation and federal prosecutors are examining people hired by hedge fund manager William Ackman about potential manipulation of Herbalife Ltd. stock, according to the Wall Street Journal.

The government is investigating whether those hired by Ackman, who has alleged the multi-level marketer is a pyramid scheme, have made false statements about Herbalife’s business model, the newspaper reported.

The Wall Street Journal added that neither Ackman nor Pershing Square has been served with a subpoena or been visited by FBI agents about the probe.

Ackman’s firm, Pershing Square Capital Management LP, has loudly touted his firm’s $1 billion short against the downtown L.A. nutritional supplements company. The two have been locked in an acrimonious public debate for the past two years.

In a statement to the Business Journal, a Pershing Square spokesman said the firm is “proud of our work over the past two years exposing the pervasive victimization by Herbalife of millions of people and would welcome any scrutiny of those efforts,” adding that Pershing has “acted lawfully in every respect.”

Herbalife Executive Vice President Alan Hoffman told the Wall Street Journal in a statement that Ackman and Pershing Square had spent tens of millions “orchestrating a false and fabricated attack against Herbalife.”

Prior to the news breaking, Herbalife shares closed up 0.45 percent at $33.25.