There’s now solid evidence that vitamin E oil found in bootleg THC vape products is behind at least some of the 2,000-plus severe vaping-related lung injuries nationwide — a finding the Centers for Disease Control and Prevention calls a “breakthrough” in the investigation.

“For the first time, we have detected a potential toxin of concern: vitamin E acetate,” Dr. Anne Schuchat, CDC’s principal deputy director, said in a call with reporters on Friday. Vitamin E acetate, an oil, is a synthetic form of vitamin E.

Investigators tested samples of fluid taken from the lungs of 29 patients with vaping illness in 10 different states, and found vitamin E acetate in all 29. No other oils, such as mineral oil or plant oils, were found at high levels.

“These findings provide direct evidence of vitamin E acetate at the primary site of injury within the lung,” said Schuchat, adding the oil had previously been detected in product samples. In September, the New York State Department of Health said that lab tests showed extremely high levels of vitamin E acetate in nearly all of the analyzed samples of products that contained cannabis.

Vitamin E is found in many common products, from food to supplements to skincare lotions, and there is no evidence it causes harm when swallowed or applied to the skin.

When inhaled, however, it appears to cause problems. One official described inhaled vitamin E acetate as a coating of honey that sticks around in the lungs.

Still, investigators say they cannot yet rule out other possible toxins or ingredients that may be causing the illnesses. There may be multiple causes, and the CDC says it will continue to test vaping aerosols.

Nearly all of the 29 patients in the CDC analysis also reported vaping THC, the marijuana ingredient that gives users a high.