The news, first reported by the Financial Times, sparked a sharp sell-off that sent the stock price down nearly 14 per cent.
One source briefed on the matter said that the investigation has been going on for some time, but declined to give further details.
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A spokesman for the Federal Bureau of Investigation in Boston declined to confirm or deny the investigation. So far no charges have been filed in the matter.
Former Herbalife distributors reached by Reuters said they had been contacted by agents who were interested in finding out more about the multilevel marketing companys business practices, including how it recruits new members into its distribution scheme.
FBI agents are also reviewing Herbalife documents obtained from former distributors, two other sources familiar with the matter said.
Herbalife said on Friday that the company has not received any inquiries from the FBI or the Justice Department about any investigation.
The company has steadfastly denied running a pyramid scheme, where distributors earn more money for recruiting new members into the scheme than they do by selling products to consumers.