Ford says the Justice Department has opened a criminal investigation into its U.S. emissions certification process.

Last September, a group of employees reported possible problems with a mathematical model used to calculate pollution and mileage, prompting Ford to hire an outside firm to run tests. And in February, Ford launched an investigation into whether it overstated mileage and understated emissions.

Ford voluntarily disclosed the matter to the Environmental Protection Agency. Ford said Friday it’s fully cooperating with all government agencies.