Vitamin E acetate, an ingredient sometimes added to THC-based products, has been identified as a "very strong culprit" in vaping-related lung injuries, health officials reported Friday.

The mysterious illness has sickened 2,051 people and killed 40, putting many patients into intensive care units, with some needing ventilators or even more desperate measures to help them breathe. Most are young, male adults or even teenagers.

Hundreds of state and federal health investigators have been deployed to find out what has caused such extensive damage to patients' lungs, which researchers have likened to the chemical burns suffered by soldiers attacked with mustard gas in World War I.

The vitamin E evidence, described Friday by the Centers for Disease Control and Prevention, is not the last word: The agency left open the possibility that other chemicals or toxins from vaping fluids or devices could also be causing the severe respiratory ailments.

The outbreak has revealed the existence of a vast, unregulated, shadowy marketplace of vaping products that are essentially a stew of unknown chemicals concocted, packed and sold by unknown manufacturers and sellers.

The new report is based on finding vitamin E acetate in fluid samples taken from the lungs of 29 patients who had the lung disease, including two who died.

"For the first time, we have detected a potential toxin of concern, vitamin E acetate, from biological samples from patients," with lung damage linked to vaping, Dr. Anne Schuchat, principal deputy director of the CDC, said at a news briefing.

The 29 patients came from 10 states scattered around the country, so the findings are considered broadly applicable and unlikely to have resulted from a single vaping product or supplier, Schuchat said.

Vitamin E acetate is sticky, like honey, and clings to lung tissue, the CDC said. Researchers do not know exactly how it harms the lungs, but studies in animals are being considered to help explain that, she said.

It is commonly used as a vitamin supplement or an ingredient in skin lotions. Makers of illicit vaping fluids sometimes add it as a thickener or to dilute the THC and increase their profits. But just because something is safe to swallow or rub on the skin does not mean it is safe to inhale. The digestive system has enzymes to break down what we ingest; lungs do not.

Although most cases of the vaping illness have been linked to illicit THC, Schuchat said that did not mean other vaping products could be considered safe. Some patients say they vaped only nicotine, and state health officials consider some of those reports reliable.