The U.S. Food and Drug Administration (FDA) announced Thursday that it has taken strong action to protect the well-being of the country’s youth by issuing warning letters to 55 retailers for selling newly regulated tobacco products — e-cigarettes, e-liquids and cigars — to underage patrons.

Last month, the FDA banned the sale of all tobacco products — including e-cigarettes — to any person under 18, adding a directive according to which retailers have to check the photo IDs of the customers under the age of 27.

“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," said Mitch Zeller, director of the FDA’s Center for Tobacco Products, in a press release. “Retailers play a vital role in keeping harmful and addictive tobacco products out of the hands of children and we urge them to take that responsibility seriously.”

E-cigarettes contain a liquid — a mix of flavoring, propylene, glycol, glycerin and nicotine — that heats up inside the device. The vapors that are released can be inhaled. The e-cigarette market has grown to $3.5 billion, Fortune reported.

The FDA discovered 55 retailers — including specialty stores, online platforms and major national chains like 7-Eleven, Chevron and Walgreens — where minors were able to purchase tobacco products like e-cigarettes, cigars, hookah and pipes. There is an attempt to market “youth-appealing” flavors like “bubble gum, cotton candy and gummy bear,” the administration found.

“It’s clear from these initial compliance checks that there’s a need for strong federal enforcement of these important youth access restrictions,” Zeller said. The agency usually issues warning letters before pursuing enforcement actions like fines or no-tobacco-sale orders in such cases.

Tobacco has been called one of the biggest public health threats by the World Health Organization, killing about 6 million people in the world every year. According to FDA and the Centers for Disease Control and Prevention data, the use of e-cigarettes has increased by more than 900 percent between 2011 and 2015 among high school students. Hookah usage has also increased significantly in this time period.