The FBI and the U.S. attorney's office are looking into whether false statements were made about the weight loss firm in order to manipulate its shares, the Wall Street Journal reported Thursday.
"I'm puzzled by the story," Ackman told CNBC. "We have not heard from the FBI or Department of Justice. If they have any questions for me I'm happy to answer them. I stand by every statement I've made about Herbalife. The company has never identified anything we've said that is false."
Herbalife shares rose about 3 percent after hours in light trading.
—CNBC's Andrew Ross Sorkin contributed to this article.