(Reuters) – AsÂ U.S health officials scramble to identify the root cause of hundreds of severe lung illnesses tied to vaping,Â one possible culpritÂ identified so farÂ isÂ a line of illicit marijuana vapeÂ productsÂ soldÂ under the brand names “Dank Vapes” and “Chronic Carts.” A study published last week in the New England Journal of Medicine found that more than half of patients with the lung illness – 24 of 41 – who were extensively interviewed in Wisconsin and Illinois reported having used the “Dank Vapes” brand. TheÂ New York State Department of Health identified “Dank Vapes” and “Chronic Carts” as products containingÂ Vitamin E acetate, a thickening agent in THC oil that has been a key focus in its investigation into the illnesses. THC is the psychoactive compound in cannabis. While Vitamin E acetate is oftenÂ applied to skin or used as a dietary supplement,Â the U.S. Food and Drug Administration hasÂ warnedÂ against inhalation because “data is limited about its effects” on the lungs. The agency has advised consumers to avoid vaping THC oils or using devices bought outside of stores. The U.S. Centers for Disease Control and Prevention (CDC), which is investigating 380 confirmed or probable cases of lung illnesses tied to vaping, said the condition has not definitively been linked to a specific product orÂ ingredient, including Vitamin E acetate. The CDC advises against using any e-cigarette or vaping products, since most of the patients interviewed used both THC and nicotine liquids, while 20 percent used only nicotine. Leading makers of nicotine e-cigarettes, including Juul Labs Inc, British American Tobacco Plc and Imperial Brands Plc, said this week their products do not contain Vitamin E compounds or THC. AVAILABLE ONLINE Packaging using the “Dank Vapes” name until recently was available on Amazon, according to caches of the product links, and a Reuters review shows they are still widely available elsewhere on the Internet. Amazon.com Inc said it took down vape paraphernalia this week in line with its policies, though the company did not specify the exact products it removed.â€¯â€¯It said THC and e-cigarettes were not, and are not, offered on its platform. One merchant that appeared to sell packaging for Dank Vapes and another brand on Amazon was known as Cart Essentials, according to the cached links, which are now defunct. Cart Essentials had 39 ratings this year, almost all of them five stars. The merchant did not immediately respond to a request for comment sent via Amazon’s platform, and Reuters could not determine who was behind the company. WHAT IS IT? Marijuana extracts used in vaping “pens” have been one of the fastest-growing segmentsâ€¯of theÂ cannabis industry in recent years. It is an easy way to use the product, can be concealed, and users can better control how much they take in, said David Downs, California bureau chief for cannabis website Leafly.com. The category grew fromÂ 2% of the legal marketÂ in 2014Â toÂ 16% last year,Â according to BDS Analytics, an industry tracker. The “Dank Vapes” brand is an illicit product that uses diluted THC oil, Downs said. Drug dealers, looking to make as much money as possible, â€¯cut THC oil with Vitamin E acetate to dilute it but make it still appear pure to consumers, Downs said. “It can cut THC oil while keeping it thick.” It is difficult to determine whether “Dank Vapes” is an actual company or a brand used by multipleÂ operators. No one responded to calls and emails sent to numbers and addresses listed on a website, dankvapes.org. New York Governor Andrew Cuomo directed the state health department earlier this weekÂ toÂ subpoenaÂ three companiesÂ heÂ saidÂ market Vitamin E acetate to vape oil manufacturers. The three companies are Honey Cut Labs LLC in Santa Monica, California;Â FloraplexÂ Terpenes in Ypsilanti, Michigan; and Mass Terpenes in Amherst, Massachusetts.Â Cuomo’s office in a statement said the thickeners were being marketed as a “cheaper, safer alternative that does not negativelyÂ impact flavoring or odor” and “can be used to cut vape products to any level of THC.” Honey Cut and Mass Terpenes could not be reached for comment, while FloraplexÂ did not respond to a request for comment. The mass-terpenes.com website had a message to customers saying it was “alarmed” by reports that Vitamin E acetate had been linked to serious lung problems and will “cooperate fully” with state and federal health authorities. It added that the company has taken steps to ensure the website has no products containing Vitamin E acetate. DISTINCT FROM AN ‘E-CIGARETTE’Â Â Illicit THC vaping pens or similar devices are distinct from e-cigarettes, such as those made by Juul Labs, which vaporize a nicotine-filled liquid. Nicotine, the addictive ingredient in cigarettes and other tobacco products, can cause serious problems for brain development in adolescents. The liquid generallyÂ also contains propylene glycol, vegetable glycerin and flavorings. Juul says that research it sponsored found that smokers who switched to its product for a few days had far less exposure to certain harmful compounds in cigarettes.Â But the long-term health effects of inhaling aerosol with these ingredients remain largely unknown, and studies have indicated potential risks for cardiovascular disease and lung health. While e-cigarettes are marketed as a means to help smokers quit or cut down, U.S health officials are concerned they are drawing a new generation into nicotine addiction.