The Food and Drug Administration (FDA) has issued warning letters to 24 websites for illegally selling e-cigarettes to minors, The Wall Street Journal reports. The agency banned e-cigarette sales to anyone under 18 years old earlier this year.

The FDA also issued warning letters to 28 retailers of e-cigarettes and cigars, and three letters to websites that sell cigars. The websites and stores must respond in 15 days with an explanation of how they plan to prevent future underage sales, or risk being fined $275.

“It’s clear from these initial compliance checks that there’s a need for strong federal enforcement of these important youth access restrictions,” Mitch Zeller, Director of the FDA’s Center for Tobacco Products, said in a news release.