Recognizing what it called “the troubling reality” that electronic cigarettes have become “wildly popular with kids,” the Food and Drug Administration on Tuesday announced a major crackdown on the vaping industry, particularly on the trendy Juul devices, aimed at curbing sales to young people.

The agency said it had started an undercover sting operation this month targeting retailers of Juuls, including gas stations, convenience stores and online retailers like eBay. So far, the F.D.A. has issued warning letters to 40 that it says violated the law preventing sales of vaping devices to anyone under 21.

The agency also demanded that Juul Labs turn over company documents about the marketing and research behind its products, including reports on focus groups and toxicology, to determine whether Juul is intentionally appealing to the youth market despite its statements to the contrary and despite knowing its addictive potential. It said it planned to issue similar letters to other manufacturers of popular vaping products as well.

“We don’t yet fully understand why these products are so popular among youth,” the agency’s commissioner, Dr. Scott Gottlieb, said in a statement. “But it’s imperative that we figure it out, and fast. These documents may help us get there.”