– New York Attorney General Eric Schneiderman told Target Corp. and three other major retailers to halt sales of store-brand herbal supplements found to lack key ingredients listed on the label, fueling calls for more regulation of the $32 billion U.S. dietary supplement industry.

In addition to Target, Schneiderman sent letters to GNC Holdings Inc., Wal-Mart Stores Inc. and Walgreens, telling them to drop products purportedly containing such herbs as echinacea, ginseng and St. John's wort.

He said his probe, conducted by a researcher who analyzed the presence of plant DNA in the products, revealed that ingredients on the labels of some supplements couldn't be verified, and that others contained undisclosed ingredients.

Target said it was pulling the products from its stores and website.

"We take these claims seriously and will continue to focus on ensuring that our products meet or exceed all relevant standards," the company said in a statement.

The U.S. Food and Drug Administration doesn't require approval for dietary supplements, as it does for pharmaceuticals. Manufacturers and distributors of the products are prohibited from selling supplements that are adulterated or misbranded, according to the agency. The agency requires notification of health problems associated with the supplements, and can order recalls of products found to violate standards.

Because vitamins and supplements aren't vetted before they hit the market, companies selling them cannot legally claim they cure or ­prevent diseases. When the FDA discovers marketing claims that cross the line, the agency's first step is to issue a warning.

Laura Brophy, a spokeswoman for Pittsburgh-based GNC, said in a statement that the nutritional products retailer stands behind the "quality, purity and potency of all ingredients listed on the labels of our private label ­products" and that the company tests all of its products using "validated and widely used testing methods."

GNC will remove products from lots named in Schneiderman's letter from stores in New York state "if required by law, not because we agree with the testing methods used to support it," she said.

Emily Hartwig, a spokeswoman for Walgreens Boots Alliance Inc., said in a statement the company will cooperate with the state attorney general and is removing the products from its shelves as it reviews the matter.

Health researchers and consumer groups have called for increased scrutiny of the market over concerns about the safety or effectiveness of the ingredients and reports of some products potentially being adulterated. Lack of oversight has "permitted the introduction of numerous supplement products, often containing unapproved active pharmaceutical ingredients, into the marketplace," a team of researchers said in a study of supplement recalls in 2013.

"When the advertised herbs aren't even in many of the products, it's a sign that this loosely regulated industry is urgently in need of reform," David Schardt, senior nutritionist at the Center for Science in the Public Interest, said in the attorney general's statement.

Claims of the herbs' effectiveness are "sketchy to begin with," said Schardt, whose Washington-based interest group has advocated for tighter restrictions for the supplement market.

James A. Schulte II of Clarkson University in Potsdam, N.Y., was hired by the attorney general to conduct the test using DNA bar coding technology, according to Schneiderman. DNA bar codes can be used to determine the exact plant species being tested, Schneiderman said.

In many instances, rather than finding evidence of species on the label, such as ginkgo biloba or ginseng, the testing turned up other ingredients such as rice, beans or a tropical houseplant, Schneiderman said. The Wal-Mart brand Spring Valley had the worst results, with only 4 percent of the tests yielding DNA matching the product label and 56 percent not yielding any plant material, according to Schneiderman.

"At Wal-Mart, we want our customers to have complete trust in the products they buy from our stores," Brian Nick, a company spokesman said in a statement. "We are immediately reaching out to the suppliers of these products to learn more information and will take appropriate action."

Staff writer Kavita Kumar contributed to this report.