WASHINGTON -- E-cigarette giant Juul Labs is facing mounting scrutiny from state law enforcement officials.

The attorneys general in Illinois and the District of Columbia are investigating how the company's vaping device became so popular with underage teens.

The company's rapid rise to the top of the multi-billion dollar U.S. e-cigarette market has been accompanied by accusations from parents, politicians and public health advocates that Juul fueled a vaping craze among high schoolers. In addition to the ongoing inquiries in Illinois and the district, which had not been publicly disclosed before, four other state attorneys general are probing or suing Juul.

Juul's top executives have disputed allegations that they've marketed their products to teens, declaring that they've taken unprecedented steps to combat underage use of its e-cigarettes.