The Food and Drug Administration is expanding its investigation into e-cigarettes.
E-cigarettes are increasingly being used as a nicotine alternative as smokers seek ways to kick their habit. They work by heating a pure liquid called e-juice -- composed of flavorings, propylene glycol, glycerin and often nicotine -- until it vaporizes. The resulting vapor is much less offensive to both smokers and non-smokers.
But their use has become controversial due to a lack of evidence on their efficacy as anti-smoking aids, warnings about possible long-term health effects and numerous studies showing that teen use is a direct gateway to traditional cigarette smoking.
Concerned with an "epidemic" surge in e-cigarette use by adolescents, the FDA took regulatory action in September against more than 1,300 US retailers and five major manufacturers for their roles in perpetuating youth access to e-cigarettes.
Now, the agency is now targeting 21 e-cigarette companies to find out if they been marketing products illegally.
The move comes less than two weeks after the FDA conducted a surprise inspection of e-cigarette maker Juul's corporate headquarters in San Francisco.
Agents seized thousands of documents, many relating to sales and marketing.
The investigations are part of the FDA's crack-down on teen e-cigarette use.
"E-cigs have become an almost ubiquitous -- and dangerous -- trend among teens," FDA commissioner Dr. Scott Gottlieb said. "The disturbing and accelerating trajectory of use we're seeing in youth, and the resulting path to addiction, must end. It's simply not tolerable."