Get breaking news alerts and special reports. The news and stories that matter, delivered weekday mornings.

An ongoing investigation of popular herbal supplements subjected to DNA testing has found numerous store brand supplements aren't what their labels claim to be, New York Attorney General Eric Schneiderman said Tuesday.

Schneiderman's office has sent letters to GNC, Target, Walmart and Walgreen Co. concerning supplements that either couldn't be verified to contain the labeled substance or that contained ingredients not listed on the label. The products include echinacea, ginseng, St. John's wort, garlic, ginkgo biloba and saw palmetto.

Overall, just 21 percent of the test results from store brand herbal supplements verified DNA from the plants listed on the labels. Schneiderman asked the companies to provide detailed information on production, processing, testing and quality control for herbal supplements sold at their stores.

"We take these issues very seriously and as a precautionary measure, we are in the process of removing these products from our shelves as we review this matter further," said Walgreen spokesman James Graham. "We intend to cooperate and work with the attorney general."

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

Walmart spokesman Brian Nick said the company is immediately reaching out to suppliers of the products and will take appropriate action.

Industry questions testing method

"We stand by the quality, purity and potency of all ingredients listed on the labels of our private label products," said GNC spokeswoman Laura Brophy. "We will certainly cooperate with the attorney general's office in all appropriate ways."

Target didn't initially respond to a request for comment.

The Council for Responsible Nutrition, a trade association representing more than 150 supplement manufacturers, criticized the method the AG's office used to test the products. "Supposed concerns about the products in question are based on a novel testing method that has been roundly criticized by botanical scientists who question whether DNA barcoding technology is an appropriate or validated test for determining the presence of herbal ingredients in finished botanical products. Processing during manufacturing of botanical supplements can remove or damage DNA; therefore while a DNA testing method can be useful in some cases, this method well may be the wrong test for these kinds of products," the CRN said in a statement on its website.

Other experts supported the results of the study, however. “This study undertaken by Attorney General Schneiderman’s office is a well-controlled, scientifically-based documentation of the outrageous degree of adulteration in the herbal supplement industry,” said Arthur P. Grollman, M.D., Professor of Pharmacological Sciences at Stony Brook University.

IN-DEPTH

-- The Associated Press and NBC News Staff