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Federal prosecutors are also involved in the investigation, and Ackman himself has not been served with a subpoena.
Ackman has been publicly shorting Herbalife since December of 2012. He has said that the multilevel marketing nutrition firm is actually a pyramid scheme, and that his hedge fund, Pershing Square, is shorting its stock to zero.
Here's the WSJ on exactly what investigators are trying to figure out in this case:
Prosecutors in the Manhattan U.S. attorney's office and New York field office of the FBI have conducted interviews and sent document requests in recent months in connection with the investigation, which is looking into whether people, including some hired by Mr. Ackman, made false statements about Herbalife's business model to regulators and others in order to spur investigations into the company and lower its stock price, the people said. Mr. Ackman's firm, Pershing Square Capital Management LP, has made a huge bet on Herbalife shares declining.
One of the people familiar with the matter said investigators are scrutinizing public statements and allegations relayed to regulators by the array of consultants and activists who have lobbied against Herbalife as well as any connections or potential collaboration between those people and Pershing Square.
Add this investigation into Ackman's involvement with Herbalife to ones already opened and ongoing by the SEC and FTC.
The question in an investigation like this is whether or not Ackman intentionally and deliberately made false statements in order to impact Herbalife's stock price. Ackman has been open about his aggressive action on this matter, admitting that he paid an Herbalife whistleblower for going public about issues within the company at one point, but that isn't considered stock manipulation.
In a statement, Pershing Square told the WSJ that it had acted transparently and fairly.
Herbalife's stock is up 5% in after hours trading.