The Food and Drug Administration, in a harsh warning letter Monday, criticized Juul Labs for illegally marketing its e-cigarettes as less harmful than regular cigarettes and ordered the company to correct the violations immediately or face tougher enforcement actions.

The agency said it acted after reviewing testimony from a recent investigation by a House Committee on Oversight and Reform subcommittee into Juul’s marketing and promotion practices. The economic and consumer policy panel, in July hearings, accused Juul of “deploying a sophisticated program” to target children and teenagers, including at schools and summer camp, as part of an effort to become the nation’s largest seller of e-cigarettes.

The FDA’s warning letter said the testimony showed that Juul marketed its vaping products as “modified risk tobacco products” — meaning the company included claims that the products “present a lower risk of tobacco-related disease or are less harmful than one or more other commercially marketed tobacco products.”

Under federal regulations, such “modified-risk claims” must be authorized by the FDA. Juul’s failure to get such authorization, the FDA said, means that the company sold and marketed “adulterated” products.

Acting FDA Commissioner Norman “Ned” Sharpless, in a statement, said “companies must demonstrate with scientific evidence that their specific product does in fact pose less risk or is less harmful” than conventional cigarettes. “JUUL has ignored the law, and very concerningly, has made some of these statements in school to our nation’s youth,” he said.

The FDA noted that the testimony at the hearing indicated that a Juul representative, speaking at a school, stated that Juul “was much safer than cigarettes” and that “FDA would approve it any day.” The panel’s investigation — based on about 55,000 documents — was detailed in a hearing and a memo compiled by staffers for the subcommittee.

Juul officials who testified at the hearing said they had ended the school programs described in the House staff memo, and that the company is no longer selling many types of e-flavored cigarettes in retail stores.

The FDA on Monday gave the company 15 days to respond to the warning letter, and demanded a description of corrective actions and plan for maintaining compliance with the law. Failure to ensure compliance “may result in the FDA initiating further action, including, but not limited to, civil money penalties, seizure, and/or injunction,” the agency said.

In a second letter to Juul, the FDA asked for documents and information about several marketing and outreach practices detailed in the congressional hearing. It said that despite commitments JUUL has made to address the surge in youth use, Juul products continue to represent “a significant proportion” of the overall use of e-cigarette products by children. “Some of this youth use appears to have been a direct result of JUUL’s product design and promotional activities and outreach efforts,” the FDA said.

The FDA is also asking JUUL to explain why it uses a formula that results in a high concentration of nicotine, which could increase the products’ addictiveness. The agency wants answers on those questions within 30 days.

A Juul spokesman said the company is reviewing the letters and “will fully cooperate.”