Illinois Attorney General Lisa Madigan's office is investigating nutrition and weight loss company Herbalife, her press secretary said on Thursday.

Madigan's office had been urged to look into the company months ago by civil rights groups who claimed that the company was a pyramid scheme targeting minorities. Hedge fund manager William Ackman was the first to call Herbalife a fraud and placed a $1 billion bet against the company in 2012.

The company denies the allegations.

"We look forward to working with the Illinois Attorney General's office to resolve the consumer complaints it has received," Herbalife said in a statement.

Madigan's office joins the Federal Trade Commission and New York Attorney General Eric Schneiderman's office in probing the company. The Federal Bureau of Investigation has reached out to former Herbalife distributors and is probing their business documents.

The company's shares remained largely unchanged immediately after the news broke. Click here to see what Herbalife's shares are doing now.