As US health officials scramble to identify the root cause of hundreds of severe lung illnesses tied to vaping, one possible culprit identified so far is a line of illicit marijuana vape products sold 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.

The New York State Department of Health identified “Dank Vapes” and “Chronic Carts” as products containing 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 often applied to skin or used as a dietary supplement, the U.S. Food and Drug Administration has warned 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.