(CBS) — Federal health officials are cracking down on underage use of a popular e-cigarette brand following months of complaints from parents, politicians and school administrators.

The FDA said Tuesday it had issued warnings to 40 retail and online stores as part of a nationwide operation against illegal sales of Juul (JOO’-uhl) to children.

Like other e-cigarettes, Juul is an electronic device that vaporizes liquid nicotine into an inhalable vapor. Thanks in part to its resemblance to a small computer flash drive, Juul has become popular with some teenagers as a discreet way to vape at school and in public.

“In some cases, our kids are trying these products and liking them without even knowing they contain nicotine,” FDA Commissioner Scott Gottlieb said in a statement.

A recent study in Tobacco Control found that 63 percent of Juul users didn’t know the product always contains nicotine.

As part of the FDA initiative, regulators are asking Juul Labs to turn over documents about the design and marketing of its devices.

Juul, which is based in San Francisco, says it monitors retailers to ensure they are following the law.

The company had nearly 47 percent of the e-cigarette market in January, according to the Winston-Salem Journal. That makes it more popular than Marlboros are among traditional cigarettes.

U.S. Senator Dick Durbin is pleased with the FDA’s regulations. He released a statement saying, “Today, the FDA heeded my call and announced positive initial steps to protect our kids from the dangers of tobacco products like the Juul e-cigarette. By taking one look at a Juul advertisement, it’s abundantly clear they have dusted off Big Tobacco’s old marketing playbook and are obviously trying to market their products to our children and teens. I’m pleased the FDA agrees that we have to do more to protect our kids from falling into a lifetime of addiction, and I continue to urge the agency to immediately use it’s existing authority to remove any Juul flavors – like their kid-popular mango flavor- that are currently on the market in violation of regulations.”

According to a press release issued by Senator Durbin’s office, Durbin and 10 of his Senate colleagues issued a letter to the FDa last week urging the agency to take immediate steps to ban kid-friendly candy and fruit flavorings that are used with e-cigarettes and cigars.

In 2016, the FDA issued a rule deeming e-cigarettes, cigars, and hookahs “tobacco products” and asserted FDA’s ability to regulate them. The FDA made a decision last year to delay its regulation of e-cigarette products and flavorings from 2018-2022. New products targeted towards youth have been able to slip by the FDA’s review, according to Senator Durbin’s office. The senators urged FDA to end the delay and begin regulation and review of e-cigarette products now.