The Food and Drug Administration is cracking down on the vaping industry, particularly on the devices being marketed towards teenagers.
The federal agency launched an undercover sting operation this month targeting retailers of Juul devices. It included gas stations, convenience stores and online retailers -- accusing them of selling products to minors.
The FDA says that electronic cigarettes have become widely popular with kids.
So far, the agency has issued warning letters to 40 retailers 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.
Those documents include reports on focus groups and toxicology, to determine whether Juul Labs is intentionally appealing to a particular market.
Nicknamed the iPhone of e-cigarettes, Juul devices resemble thumb drives and produce a small plume and smell like fruit or other flavorings.
Some say the smell is so concealable, students can even vape in class without it being noticed.
In a statement, Juul Labs said it agreed with the FDA, saying illegal sales of the company's product to minors is unacceptable.
The company says it is working with the FDA, lawmakers and parents to combat underage vaping.
Meanwhile, the FDA says it plans to issue similar letters to other manufacturers of popular vaping products.