Ford says the Justice Department has opened a criminal investigation into its U.S. certification process for measuring vehicle emissions and gas mileage.

The automaker said in a regulatory filing Friday the "matter currently focuses on issues relating to road load estimations, including analytical modeling and coastdown testing."

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 gas mileage and understated emissions from a wide range of vehicles.

Ford Motor Co. voluntarily disclosed the matter to the U.S. Environmental Protection Agency and the California Air Resources Board in February. It has also notified a number of other state and federal agencies.

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Ford said Friday it's fully cooperating with all government agencies. Because the Justice Department is still in the early stages of its investigation, Ford told investors the company "cannot provide assurance that it will not have a material adverse effect on us."

Wall Street initially shrugged off the DOJ news. The price of Ford stock jumped nearly 10% Friday morning, to about $10.30 a share, after the company reported especially strong North American sales of its F-series pickup trucks.

Automakers face emissions cheating allegations

Ford is the latest automaker tangled up in allegations of emissions testing misconduct. In January, Fiat Chrysler agreed to pay $800 million to pay civil penalties and compensate Jeep and Ram owners, after it was alleged the company installed software that allowed vehicles to cheat on U.S. emissions testing.

Volkswagen infamously paid $14.7 billion in civil penalties and $3 billion in criminal fines after settling a widespread emissions-cheating scandal with the U.S. Environmental Protection Agency. The German auto company was accused of using what are called "defeat devices" that turned on during emissions testing, but not in normal driving.

Ford emphasized Friday that its own internal investigation does not involve "defeat devices."