General Nutrition Corporation or GNC is on the hot seat as it is being accused of selling dietary supplements incorporated with illegal additives.  Along with other sellers, GNC received notification letters from the New York Attorney General's office saying that it had been unable to verify several supplements contained in the label or some ingredients are not listed in the label.

State Attorney General Ellen Rosenblum filed a case against the nutrition store on Thursday.  It accuses GNC that it knows it's sold products contain substances that resembles amphetamine substance that isn't allowed for diet pills in the U.S., leaving it off the label, as reported by CNN Money.

Another complaint claims that some GNC supplements contained an illegal synthetic chemical that's used to cure neurological disorders in some countries. The lawsuit follows an investigation aided by the FDA, according to the Oregon Department of Justice.

"It is scary to know that certain products sold by GNC contain an ingredient that is not even labeled -- let alone approved in the United States," Rosenblum said in a statement.

GNC denied the accusations and said it will "vigorously defend against these allegations." The company also said when the FDA banned the substances, "GNC promptly took action to remove from sale all products containing those ingredients," it said in a statement Thursday.

The supplement store's stock plummeted more than 14% Thursday after a brief trading stop ahead of the company response to the lawsuit.

The supplement industry has been recently under heavy surveillance.  New York's AG went after GNC in February, demanding the company together with Target, Walmart, and Walgreens to refrain from selling herbal supplements that do not contain herbs.  In March, GNC agreed to boost quality control and testing of herbal supplements.

According to CNBC, GNC is selling thousands of units of 22 workout and fat-burner supplement products that contained picamilon, a Russian prescription drug used to treat neurological conditions.  Internal company records proved that GNC official knew since 2007 that this type of drug is not natural and should not be sold as a supplement.

Other supplements that the store sells contained synthetic chemicals known as BMPEA, Beta-methylphenethylamine and by numerous other chemical names.  Among other evidence of misdeed, the attorney general's office sees emails circulated two years ago among top GNC executives in the wake of a 2013 USA TODAY article about BMPEA in supplements.  "The USA Today article stimulated significant concern and discussion within GNC," the suit says, but still the company continued to sell BMPEA-containing products until earlier this year.

Federal and state officials have been implementing disciplinary action for years on allegedly natural supplements consisting of hidden drug ingredients.  The FDA says weight loss, bodybuilding and male enhancement products are the biggest violators, commonly being marked as containing only plant products, but the truth is it's been loaded with prescription drug ingredients, NBC News reported.

The Food and Drug Administration sent warning notifications to five companies in April informing them that eight of their products are in the list of BMPEA and orders them to inhibit from selling those products.  The companies affirmed a plant known as Acacia rigidula was the source of the chemical, but FDA confirms the plant does not consist of BMPEA.

GNC along with the other companies had known the link between BMPEA and acacia.