Illinois Attorney General Lisa Madigan is probing Herbalife, Reuters' Svea Herbst-Bayliss reports citing a press official for the state's AG.
Herbalife is a multi-level marketing company that sells weight loss shakes and nutritional supplements.
The stock fell about 2 percent following the news. It was last off about 0.5%.
Herbalife has been at the cross-hairs of a nasty battle between hedge fund billionaires.
Bill Ackman, who runs Pershing Square, is shorting $1 billion worth of Herbalife. Ackman believes the company operates as a "pyramid scheme". He has a price target of $0. In other words, it's Ackman's contention that regulators, specifically the Federal Trade Commission, will shut the company down.
Ackman's arch-nemesis, billionaire investor Carl Icahn, snapped up a large position in the stock shortly after Ackman publicly revealed his short position. He owns 16.8% of Herbalife's shares.
Last month, Herbalife confirmed last month that it's under investigation by the FTC.
The Financial Times reported last Friday that the company was being probed by the DOJ and the FBI. The company said in statement that they had no knowledge of any ongoing investigation by the DOJ or the FBI.
The New York Post also reported this week that New York Attorney General Eric Schneiderman's office is probing Herbalife.