Federal health authorities say vaping giant Juul Labs illegally pitched its electronic cigarettes as a safer alternative to smoking, including in a presentation at schools.

The Food and Drug Administration on Monday issued a stern warning letter to the company, flagging claims by Juul including that its products are "much safer than cigarettes."

"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," said Acting FDA Commissioner Ned Sharpless in a statement. "JuuL has ignored the law, and very concerningly, has made some of these statements in school to our nation's youth."

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The FDA has been investigating Juul for months but had not previously warned the company. Juul, which has three-quarters of the vaping market, has also been the target of lawsuits over allegations that it targeted teens with deceptive marketing practices.

The FDA letter highlights an incident that two New York high school students recounted during a congressional hearing in July. The teens testified that a Juul representative was invited to address the school on addiction issues. During the presentation, the representative told students the company's product was "totally safe."

In marketing materials, Juul has also referred to its products as "99% safer than cigarettes, "much safer" than cigarettes, "totally safe" and "a safer alternative than smoking cigarettes," according to the FDA letter.

Juul said it discontinued its school programs last September. Responding Monday to the FDA's letter, Juul said in a statement that "We are reviewing the letters and will fully cooperate."

Vaping has recently been linked to at least five deaths and 450 illnesses nationwide, Prompting the Centers for Disease Control and Prevention to warn people "to consider not vaping pending further investigation."

Last week, Michigan became the first state to ban sales of flavored e-cigarettes. San Francisco has also banned sales of e-cigarettes that are not FDA approved.