Juul illegally marketed its e-cigarettes as less harmful than tobacco cigarettes, and must find a way to correct that violation, according to a warning letter from the Food and Drug Administration (FDA).

E-cigarettes like Juuls, which heat liquid nicotine instead of burning tobacco, are thought to be less dangerous than cigarettes because they produce fewer cancer-causing chemicals. But “regardless of where products like e-cigarettes fall on the continuum of tobacco product risk, the law is clear that, before marketing tobacco products for reduced risk, companies must demonstrate with scientific evidence that their specific product does in fact pose less risk or is less harmful,” Acting FDA Commissioner Dr. Ned Sharpless said in a statement Monday. “JUUL has ignored the law, and very concerningly, has made some of these statements in school to our nation’s youth.”

E-cigarettes, including the ones made by Juul, are not currently FDA-approved. Juul has until spring 2020 to apply for approval as a new tobacco product.

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“We are reviewing the letters and will fully cooperate,” a Juul spokesperson told TIME. The letter requires Juul to submit a plan of correction within 15 business days. If it fails to comply with FDA policy, the agency could resort to fines or injunctions against the company.

The warning letter cites several accusations made in a July Congressional hearing relating to Juul—among them, the allegation that a Juul representative on a school visit told students that the product “was much safer than cigarettes” and “totally safe.” A letter from the CEO posted on Juul’s website also argued that Juul devices “deliver smokers the satisfaction that they want without the combustion and the harm associated with it,” according to the FDA’s statement.

The FDA is also investigating Juul’s use of nicotine salts, a formulation that makes nicotine more palatable to consume and may allow users to take in higher doses at a time, and its high nicotine concentrations, which may be particularly addictive.

As by far the most popular e-cigarette on the market, Juul has been at the center of the FDA’s efforts to curb an epidemic of underage vaping. Last year, the agency carried out an unannounced inspection of its San Francisco headquarters and seized thousands of pages of documents related to its marketing and advertising activity, some of which has been criticized for illegally targeting youth. At the end of 2018, just ahead of proposed FDA regulations to limit the sale of flavored vaping products, Juul elected to stop selling many of its flavored pods in most retail stores, and to suspend its U.S. Instagram and Facebook accounts.

The FDA’s warning comes amid mounting reports of serious lung diseases associated with vaping. Many cases, however, appear to be related to vaping marijuana, not the nicotine contained in Juul pods.