A patient has died after developing a severe respiratory disease due to vaping in the first such death in the US, say health officials.
It comes as experts investigate a mystery lung disease across the US that is linked to use of e-cigarettes.
The Centers for Disease Control and Prevention (CDC) said there were 193 "potential cases" in 22 US states.
Many of the cases involve vaping THC, the main active compound in cannabis, CDC experts said.
The cases were reported over the course of two months between 28 June and 20 August.
The person who died was "hospitalized with unexplained illness after reported vaping or e-cigarette use", Dr Jennifer Layden, the chief medical officer and state epidemiologist in Illinois, said.
CDC director Robert Redfield said: "We are saddened to hear of the first death related to the outbreak of severe lung disease in those who use e-cigarette or 'vaping' devices."
He added: "This tragic death in Illinois reinforces the serious risks associated with e-cigarette products."
What is the illness?
The cause of the mystery illness has not been identified, but all involve vaping in some form.
"In many cases, patients have acknowledged recent use of THC-containing product," the CDC's head of non-infectious diseases, Dr Ileana Arias, said.
Those affected had symptoms including coughing, shortness of breath and fatigue as well as some cases of vomiting and diarrhoea. There is no evidence of an infectious disease - such as a virus or bacteria - being responsible.
But there is much that remains a mystery.
"It isn't clear if these cases have a common cause or if they are different diseases with similar presentations," Dr Arias said.
There have been 22 cases in the state of Illinois, with patients ranging from 17 to 38 years old.
The 22 states affected are largely in the centre and north-east of the country, from Minnesota to North Carolina, though cases have also been reported in California, Texas, and New Mexico.
Officials have ordered laboratory tests of vaping liquid samples in a bid to identify any harmful compounds.
A "black market" is known to exist for THC-containing vape cartridges, which are sold legally through medical marijuana dispensaries in some states.
Users in online communities have warned of the dangers of unregulated fake cartridges.
Is regular vaping safe?
No cause for the mystery illness has been identified - and the link to THC products is not clear yet either.
Mitch Zeller of the US Food and Drug administration said it was important to understand that "we find ourselves in the early stages of these investigations trying to piece together the facts."
He said the FDA was exploring whether the products were used as intended or being modified by adding something to them.
The president of the American Vaping Association, Gregory Conley, said in a statement on Thursday that he was "confident" the illnesses were being caused by devices containing cannabis or other synthetic drugs, not nicotine.
But Dr Brian King from the CDC's office on smoking and health warned against thinking of vaping as completely safe.
"We do know that e-cigarettes do not emit a harmless aerosol," he said.
"There's a variety of harmful ingredients identified, including things like ultrafine particulates, heavy metals like lead and cancer causing chemicals," he said, along with diacetyl - a flavouring used to give a "buttery" taste that has been linked to "severe respiratory illness".
He also warned of the possibility that similar cases had been happening for a long time, but the connection between them had been missed.
"It's possible that the reported cases could have been occurring before this investigation was even initiated," he said.
Dr King said it was possible "we weren't necessarily capturing them, but now there's increased diligence in terms of the current investigation that we're better able to".
At least two people have previously died in the US after their e-cigarette exploded in their face.