The U.S. Justice Department sued Fiat Chrysler Automobiles NV and alleged it used illegal software to cheat on government emissions tests, escalating a battle over the company’s diesel engines.

In a civil lawsuit filed Tuesday in a Detroit federal court, U.S. prosecutors said the Italian-American auto maker used defeat-device software that allowed nearly 104,000 2014-2016 model-year Jeep Grand Cherokee sport-utility vehicles and Ram pickup trucks with diesel engines to pass government emissions tests and then pollute far beyond legal limits on the road.

The suit accuses Fiat Chrysler of violating the federal Clean Air Act and seeks injunctive relief and civil penalties. The suit also named as a defendant VM Motori SpA, a company owned by Fiat Chrysler that designed the diesel engines.

Fiat Chrysler shares fell more than 4% in trading on the New York Stock Exchange, erasing gains earlier in the day.

Fiat Chrysler said it was “disappointed” the Justice Department filed the lawsuit and “intends to defend itself vigorously, particularly against any claims that the company engaged in any deliberate scheme to install defeat devices to cheat U.S. emissions tests.”