Doctors who examined lung tissue from patients suffering from vaping-related lung illnesses report the damage resembles exposure to toxic chemicals.

In this week’s New England Journal of Medicine, doctors from the Mayo Clinic write that the injuries look like those seen in people exposed to poisons such as mustard gas, a chemical weapon used in World War I, The New York Times reports.

The doctors examined lung tissue from 17 patients who became ill after vaping nicotine or marijuana products. Two of the patients died. About 70 percent had a history of vaping marijuana or cannabis oils, the article notes.

Scientists initially thought vaping-related lung injuries were caused by the oils being vaped, such as THC oil or vitamin E oil. But the Mayo Clinic researchers said they did not see any signs of oil accumulating in lung tissue. Lead researcher Brandon Larson said it is too soon to know whether the survivors’ lungs will fully recover.