The Food and Drug Administration (FDA) sent warning letters to at least 24 websites and 28 online e-cigarette retailers, according to the Wall Street Journal, detailing its new rules regarding selling or marketing to minors and possible penalties if changes aren’t made to sites within 15 days.

“We’re helping protect the health of America’s youth by enforcing restrictions that make it illegal to sell tobacco products to minors – including e-cigarettes, e-liquids and cigars,” FDA Center for Tobacco Products director Mitch Zeller wrote in a press release. “It’s clear from these initial compliance checks that there’s a need for strong federal enforcement of these important youth access restrictions.”

None of the violators were vape shops, according to the Journal. Retailers who did receive warnings included gas stations, convenience stores and drugstores who were cited most often for selling cigars brands like Swisher Sweets, which is owned by Swisher International (SWSH), and Black & Mild, which is owned by Altria (MO), the nation’s largest tobacco company, the Journal reports.

The crackdown comes less than six weeks after FDA banned e-cigarette sales to minors after finalizing new regulations for e-cigs, hookah, and cigars in May.

Research released from FDA and the Centers for Disease Control & Prevention (CDC) earlier this year revealed current e-cigarette use among high school students increased by more than 900% between 2011 and 2015 while conventional cigarette smoking among American teens is on the decline.

E-cigarettes’ have grown into a $3.5 billion market, but public health officials have questioned their safety even though manufacturers claim they are healthier alternatives to traditional tobacco.

Health advocates fear that perception is drawing young Americans to use the trendy devices, which often come with flavored nicotine.

FDA research shows some vaping products contain high amounts of carcinogens like formaldehyde and acetaldehye.