The Food and Drug Administration (FDA) is taking e-cigarette giant Juul to task for violating federal regulations with its marketing. According to a warning letter from the FDA, Juul improperly sold its vaping products as safer alternatives to traditional tobacco cigarettes.

According to federal law, companies like Juul are not allowed to market products as safer than cigarettes without proving these claims to the FDA. The administration highlighted specific instances where Juul made these claims, including a school presentation at which a company representative told students that their products were safer than cigarettes and “totally harmless.”

The FDA also pointed to a statement from the company’s CEO that could be found on the Juul website for a time which claimed that their products deliver nicotine without “the combustion and harm associated with [cigarettes].”

This warning also came after the FDA discovered over 400 cases of lung disease linked to vaping, including five potential deaths, per the New York Times. Investigators haven’t yet determined the exact cause of these cases, but it has been suggested that they are linked to the use of products containing cannabis and nicotine.

“[Juul] has engaged in labeling, advertising, and/or other activities directed to consumers, in which JUUL explicitly and/or implicitly has represented that JUUL products are free of a substance,” the letter reads. “Have a reduced level of or exposure to a substance, and/or that JUUL products present a lower risk of tobacco-related disease or are less harmful than one or more other commercially marketed tobacco products.”

Juul has said that they are looking at the letter and will fully cooperate with the FDA.