The recent outbreak of vaping-related lung injuries was most likely caused by direct exposure to toxic chemical fumes, according to a significant Mayo Clinic finding into the recent outbreak which has left more than 800 people hospitalized and 17 others dead.

Mayo researchers in a report published Wednesday in the New England Journal of Medicine said a first-ever examination of 17 biopsies from patients afflicted with vaping-related lung injuries showed no evidence of oil or lipid contamination which had been the suspected culprit behind the outbreak.

The Mayo doctors, however, say they found the lung injuries consistent with ‘direct lung toxicity from an inhaled noxious agent or agents,’ similar to when someone is exposed to a toxic chemical spill, or when World War I soldiers breathed in deadly mustard gas, reports the Star Tribune.