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

GNC, Target, Walmart and Walgreen Co. sold supplements that either couldn't be verified to contain the labeled substance or that contained ingredients not listed on the label, Schneiderman's office said.

The supplements, including echinacea, ginseng, St. John's wort, garlic, ginkgo biloba and saw palmetto, were contaminated with substances including rice, beans, pine, citrus, asparagus, primrose, wheat, houseplant and wild carrot. In many cases, unlisted contaminants were the only plant material found in the product samples.

Overall, 21 percent of the test results from store brand herbal supplements contained DNA from the plants listed on the labels. The retailer with the poorest showing was Walmart, where 4 percent of the products tested showed DNA from the plants listed on the labels.

"This investigation makes one thing abundantly clear: The old adage 'buyer beware' may be especially true for consumers of herbal supplements," Schneiderman said.

Schneiderman asked the companies to provide detailed information on production, processing, testing and quality control for herbal supplements sold at their stores.

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

Walgreen and GNC pledged to cooperate with the attorney general.

"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," Walgreen spokesman James Graham said.

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

"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.

The investigation looked at six herbal supplements sold at stores across the state. Testing was performed by an expert in DNA barcoding technology, James Schulte II of Clarkson University in Potsdam. The DNA tests were performed on three to four samples of each of the supplements purchased. Each sample was tested five times. Overall, 390 tests involving 78 samples were performed.

Michael McGuffin, president of the American Herbal Products Association, which represents the herbal industry, called DNA testing "an emerging technology that has the potential to be useful in the future when it has been rigorously tested and validated." He said identification of an herb through DNA testing must be confirmed with established analytical tools that herbal experts use, such as chromatography or microscopy.

The U.S. Food and Drug Administration requires companies to verify their products are safe and properly labeled. But supplements aren't subjected to the rigorous evaluation process used for drugs.

If a manufacturer fails to identify all the ingredients on an herbal product's label, a consumer with allergies or who is taking medication for an unrelated illness could risk serious health issues every time a contaminated herbal supplement is ingested.

A DNA study conducted by the University of Guelph in 2013 also found contamination and substitution in herbal products in most of the products tested. One product labeled as St. John's wort, often used to treat depression, contained Senna alexandrina, a plant with laxative properties. One ginkgo product was contaminated with black walnut, which could endanger people with nut allergies.

A 2013 study from the Canadian Institutes of Health Research estimated there are about 65,000 dietary supplements on the market consumed by more than 150 million Americans.

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(Photo: Science Photo Library/Corbis)