Ninth-graders who used electronic cigarettes were more likely to smoke cigarettes, cigars or hookahs than peers who never tried the battery-powered devices, a study in the Journal of the American Medical Association found.

The research is some of the first to address fear among health officials that e-cigarettes could become a gateway to traditional cigarettes. The results come as the largely unregulated $3.5 billion e-cigarette industry faces mounting criticism from health groups and lawmakers concerned about teens using the devices, which heat liquid nicotine into vapor.

The study focused on ninth-graders at 10 public schools in Los Angeles who had tried e-cigarettes before the fall of 2013. Researchers surveyed those students in the spring of 2014 and fall of 2014, and discovered that they were about 2½ times as likely as their peers to have smoked traditional cigarettes, five times as likely to have smoked cigars, and three times as likely to have smoked hookahs.

Researchers didn’t determine if using e-cigarettes led teens to try smoking. But Adam Leventhal, who led the study as director of the University of Southern California Health, Emotion and Addiction Laboratory, said the results showed that a “disproportionate number of smokers were e-cigarette users” first.

Because there is no federal law restricting e-cig sales to minors, more than 45 states have passed their own rules prohibiting sales of the devices to anyone under 18, the minimum purchase age for cigarettes.