Ford has said the Justice Department has opened a criminal investigation into its US emissions certification process.
The carmaker said in a regulatory filing the “matter currently focuses on issues relating to road load estimations, including analytical modelling 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 voluntarily disclosed the matter to the Environmental Protection Agency and the California Air Resources Board in February.
It has also notified a number of other state and US federal agencies.
Ford said on Friday it is fully cooperating with all government agencies.
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules here