DETROIT (AP) — Hyundai and Kia overstated the gas mileage on 900,000 vehicles in the past three years, a discovery that could result in sanctions from the U.S. government and millions of dollars in payments to car owners.

The inflated figures were uncovered by the Environmental Protection Agency in an audit of gas mileage tests by the two South Korean automakers. The agency, which monitors fuel economy, said Friday that it's investigating how the companies came up with their numbers.

The EPA found inflated gas mileage on 13 models from the 2011 through 2013 model years, including Hyundai's Elantra and Tucson, and Kia's Sportage and Rio. The window sticker mileages were overstated on about one-third of the cars sold by the companies during the three years.