Juul Labs Inc., the largest maker of e-cigarette products, is facing a federal criminal probe, according to a published report Monday.

The U.S. attorney's office for the Northern District of California is conducting the investigation, according to a story in the online edition of The Wall Street Journal. Details of the probe haven't yet emerged, according to the report.

E-cigarettes have come under increasing regulatory focus as millions of young people have begun using the products, lured by candy and fruit flavored vaping products. Several recent deaths linked to e-cigarettes have prompted additional calls for their regulation.

Altria Group Inc. (MO) - Get Report , which makes Marlboro cigarettes, took a 35% stake in San Francisco-based Juul last year.