• Nearly a dozen Democratic senators sent a scathing letter to Juul on Monday.
  • The senators will investigate Juul's marketing tactics.
  • They're also looking into Juul selling a minority stake to Marlboro maker Altria. 

Nearly a dozen Democratic senators sent a scathing letter to Juul on Monday asking the company to answer questions about its marketing practices and deal with Marlboro maker Altria.

Eleven Democratic senators — including Senate Democratic Whip Dick Durbin, presidential candidate Elizabeth Warren, Ron Wyden and Richard Blumenthal — said they will investigate Juul's marketing practices. They will also look into Juul's sale of a 35 percent stake in itself to tobacco giant Altria and whether Juul is violating regulations or commitments it made to the Food and Drug Administration.

Juul has defended its decision to take money from Altria as a way to reach more smokers. Altria's Marlboro is the nation's best-selling cigarette brand. The idea is that Altria can help Juul switch more adult smokers from Marlboro to Juul. E-cigarettes are less harmful than cigarettes, though they may pose their own long-term health risks, the National Academies of Sciences, Engineering, and Medicine has concluded.

"While JUUL has promised to address youth vaping through its modest voluntary efforts, by accepting $12.8 billion from Altria—a tobacco giant with such a disturbing record of deceptive marketing to hook children onto cigarettes —JUUL has lost what little remaining credibility the company had when it claimed to care about the public health," the senators wrote.

For more on investing in health care innovation, click here to join CNBC at our Healthy Returns Summit in New York City on May 21.

Public health officials blame Juul for a spike in teen vaping. Critics point to past marketing materials that use bright colors and attractive young models as evidence the company intentionally targeted young people, a claim Juul vehemently denies.

A Juul spokesman said in a statement that the company welcomes "the opportunity to share information" about its efforts to stop teens from using its products and getting them in the hands of adults.

"We agree that companies such as ours must step up with meaningful measures to limit access and appeal of vapor products to young people," he said. "That's exactly what we've done, and we will do more to combat teen use to save the harm-reduction opportunity for the 34 million adult smokers in the United States."

Over the past year, Juul has taken a number of actions to try to stop kids from using its products, including removing its fruity flavored e-cigarettes from store shelves. Still, former FDA Commissioner Scott Gottlieb accused Juul and Altria of reneging on its promises when they struck a deal in December.

Now, the two companies must also answer to Congress.

"The corporate marriage between two companies that have been the most prolific at marketing highly addictive nicotine products to children is alarming from a public health standpoint and demonstrates, yet again, that JUUL is more interested in padding its profit margins than protecting our nation's health," the senators wrote.

The Juul spokesman said in the statement that "the Altria investment will help us switch adult smokers off of combustible cigarettes by helping us get our product in their hands." Altria spokesman Steve Callahan said its stake in Juul represents "a significant investment" toward the goal of achieving tobacco harm reduction.

"We understand that the future harm reduction potential of products like these is threatened by youth usage of e-vapor products. Both Altria and JUUL are committed to being part of the solution to the youth vaping problem," Callahan said, noting the companies' efforts to raise the minimum tobacco buying age to 21.

Read the senators' full letter to Juul: