FCA CEO Sergio Marchionne said earlier this year that the automaker wasn't attempting to cheat on its diesel emissions tests.

Looks like Fiat Chrysler’s best efforts weren’t enough as the U.S. Justice Department filed a lawsuit accusing the automaker of illegally using software to bypass emission controls in 104,000 diesel-equipped Jeep Grand Cherokees and Dodge Ram 1500 trucks sold since 2014.

Filed in U.S. District court in Detroit, the suit is a procedural step designed to turn up the pressure on Fiat Chrysler, according to Reuters. While Volkswagen AG has been dealing with its diesel emissions problem for some time — and at a cost in excess of $25 billion — this suit may be a signal that more makers should be concerned in the U.S. and abroad.

The automaker has been working with federal regulators to resolve the issue and just yesterday appeared to believe it was on the path to avoiding a lawsuit from the Justice Dept. and potentially $6 billion in fines.

“FCA US is currently reviewing the complaint, but is disappointed that the DOJ-ENRD has chosen to file this lawsuit,” FCA said in a released statement. “The company 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.”

(Diesel fix could help FCA avoid showdown with feds in court. Click Here for the story.)

While FCA was quick to go on the offensive, protesting its innocence, others were less generous in their categorization of FCA’s culpability in its diesel emission problem.

The U.S. Justice Dept. alleges in its lawsuit that FCA cheated on diesel emission testing for 104,000 Jeep Grand Cherokees and Dodge Ram 1500 trucks.

“Fiat Chrysler joins the long list of automakers who have put our clean air and health of our families in the backseat by cheating on emissions tests,” said Michael Brune, executive director, Sierra Club, in a statement.

“No profit margin is worth poisoning our neighborhoods with toxic smog, yet Fiat Chrysler’s deceit is as dangerous as the sickening smog these vehicles leave behind. Consumers deserve corporate accountability and clean cars that put our health first.”

Brune renewed the Sierra Club’s call for a move to electric vehicles “because without tailpipes its difficult to cheat on tailpipe emissions tests.”

(Click Here to see details about France investigating FCA for diesel emission violations.)

The EPA accused FCA of using the defeat device last January, just before the end of the Obama Administration. That was 16 months after the agency leveled charges against Volkswagen.

The German automaker has since agreed to fines, penalties and other actions that will come to as much as $30 billion – about a third of that covering the cost of buying back most of the vehicles using the smaller engine.

The EPA and CARB recently agreed to allow VW to resume selling some 2015 models that it can bring into compliance with emissions standards.

Among domestic automakers, FCA has been the most aggressive at returning to the diesel market the Detroit Big Three largely abandoned in the 1980s.

Even with a software fix for the diesels, it is unclear whether the EPA and CARB will then let the issue wrap up – though under the Trump Administration some observers expect regulators to be less likely to pursue additional fines or the criminal penalties that VW faced.

(Fiat Chrysler subpoenaed by feds of diesel emissions allegations. Click Here for the story.)

FCA said it is already using the modified software on its 2017 diesel models. The maker says it remains committed to the technology, though some other manufacturers seem less sure. Mercedes-Benz has decided not to offer any diesels in the U.S. this year and is studying whether to return to the market in the future.