For months, U.S. health officials have scrambled to determine why more than 800 people have developed lung illnesses and at least 12 people have died due to problems connected to vaping, but reached frustratingly few conclusions. The Centers for Disease Control and Prevention (CDC) last week made its strongest statement yet, saying it believes products containing THC, the main psychoactive compound in marijuana, play a role in the outbreak—but even still, no single substance, product or device has been uncovered as a cause of all cases.

Independent analysis conducted by a group of Mayo Clinic researchers may bring investigators closer to a conclusion. In what the authors say is the first study of lung biopsies associated with the outbreak, researchers analyzed 17 tissue samples taken from patients who got sick after vaping, about a dozen of whom had vaped products containing marijuana or cannabis oils, and two of whom died. The researchers found signs of lung damage similar to what would occur after exposure to toxic chemicals, gases or other substances, according to correspondence published in the New England Journal of Medicine on Oct. 2.

“What we see with these vaping cases is a kind of severe chemical injury that I’ve never seen before in a tobacco smoker or a traditional marijuana smoker,” letter co-author and Mayo Clinic pathologist Dr. Brandon Larsen said in a video released by the Mayo Clinic. “I think we’ve only seen the tip of the iceberg.”

The results of 17 biopsies don’t necessarily apply to an entire 800-person outbreak, and the researchers weren’t able to determine precisely which chemicals could be causing the lung damage they observed. Nonetheless, their finding is significant, because it weakens one prominent theory about what’s causing vaping-related illnesses: lipid accumulation resulting from the inhalation of fatty substances, like oils in which the flavoring agents and chemical compounds in vaping products are sometimes suspended.

The new research doesn’t completely disprove the idea that inhaling these substances is causing lung damage, but it does question that theory. Instead of lipid accumulation, the researchers found damage to the airways that was indicative of pneumonia, and appeared to have been contracted after inhaling “one or more toxic substances.”

The study is an important step forward, but its researchers caution that “the agents responsible remain unknown.” As investigations continue, a number of states have enacted bans on vaping products, and CDC officials have urged Americans not to use e-cigarettes at least until it determines what is causing the growing number of illnesses.

Write to Jamie Ducharme at jamie.ducharme@time.com.