Washington, United States — The death toll in the United States from illnesses linked to e-cigarette use has risen to at least 19, health authorities say, as 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, and are pursuing multiple lines of investigation.

A report by clinicians in North Carolina last month pointed to the inhalation of fatty substances from aerosolized oils, but a new study by the Mayo Clinic published this week found patients’ lungs had been exposed to noxious fumes.

The Centers for Disease Control and Prevention said Thursday that 18 deaths in 15 states had now been positively linked to vaping, from a total of 1,080 cases of people sickened — a jump of 275 since last week.

Connecticut officials also announced the first death in the state, bringing the total to at least 19.

The CDC attributed the sharp increase to a combination of new patients becoming ill in the past two weeks and recent reporting of previously identified patients.

“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 percent reported using tetrahydrocannabinol (THC), the primary psychoactive substance of marijuana, with or without nicotine products.

Another 37 percent reported exclusive use of THC products, and 17 percent said they had only used nicotine-containing products.

About 70 percent of patients are male, and 80 percent 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.

Initially conceived as a smoking cessation device, e-cigarette use has skyrocketed among teens, with preliminary official data for 2019 showing more than a quarter of high school students using e-cigarettes in the past 30 days.

They were until recently perceived as a less harmful alternative to smoking because they do not contain the 7,000 chemicals in cigarettes, dozens of which are known to cause cancer.

Only one case of lung injury has been reported abroad, making the outbreak more mysterious still.

Canadian authorities said in September a youth had been hospitalized, but so far no other countries have reported anything similar.

Public and political opinion appears to be hardening, however, with the administration of US President Donald Trump announcing in September that it would ban in the coming months flavored e-cigarette products, which are particularly attractive to young people.

India has issued an outright ban on all e-cigarette products, as has the US state of Massachusetts.