The death toll in the United States from illnesses linked to e-cigarette use has reportedly risen to at least 19.

US health authorities say more than 1,000 others have suffered lung injuries probably linked to vaping. Officials have yet to identify the cause for the outbreak, which dates back to March. A report by clinicians in North Carolina in September pointed to the inhalation of fatty substances from aerosolized oils, but a new study by the Mayo Clinic published this month found patients’ lungs had been exposed to noxious fumes.

The Centers for Disease Control and Prevention said last week that 18 deaths in 15 states in the US had now been positively linked to vaping. Connecticut officials also announced the first death in the state, bringing the total to at least 19.

“I think we really have the feeling right now that there may be a lot of different nasty things in e-cigarette or vaping products, and they may cause different harms in the lung,” Anne Schuchat, a senior official with the Centers for Disease Control and Prevention (CDC), said in a call with reporters.

Among a group of 578 patients interviewed on substances they had used, 78 per cent reported using tetrahydrocannabinol (THC), the primary psychoactive substance of marijuana, with or without nicotine products. Another 37 per cent reported exclusive use of THC products, and 17 per cent said they had only used nicotine-containing products. About 70 per cent of patients are male, and 80 per cent are under 35 years old.

E-cigarettes have been available in the US since 2006. It is not clear whether the outbreak is only happening now—or if there were cases earlier that were wrongly diagnosed.