Juul's challenges mounted Monday as the Food and Drug Administration issued a warning letter to the e-cigarette maker.

The FDA warned Juul it was breaking the law by marketing its vaping products as less harmful than traditional tobacco products.

In particular, Juul is breaking the law by selling and distributing its e-cigarettes "as modified risk tobacco products without an FDA order in effect that permits such sale or distribution," the FDA's letter stated.

Shares of Altria (MO) - Get Report , which owns 35% of Juul, were slow to see an impact from the FDA crackdown, rising 0.25% to $43.96.

"We are reviewing the letters and will fully cooperate," a spokesperson for Juul Labs said in a statement  

While the FDA letter makes no mention of the issue, e-cigarette makers have come under mounting pressure in the last few weeks amid hundreds of mysterious lung ailments around the country that have been tied to vaping, resulting in three deaths. Some of those who fell ill reported vaping nicotine and THC, with a smaller number just vaped nicotine.  

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

The letter cited evidence aired at the House Committee on Oversight and Reform's Economic and Consumer Policy subcommittee hearing in July that included questionable statements by Juul executives.

The panel heard testimony that, among other things, a Juul representative speaking with students had stated Juul "was much safer than cigarettes" and that "FDA would approve it any day."[1]

British American Tobacco, which makes rival vaping products, released a statement saying the company is "monitoring the situation in the USA and will work closely with all relevant Public Health Authorities around the world such as the FDA, the Centre for Disease Control and Prevention and Public Health England."