“It’s pretty much been a disaster,” Mr. Ramsey said. “This is evidence that they couldn’t actually make it work, but continued to sell them and market them for years.”

Fifteen years ago, Mercedes-Benz, BMW and Volkswagen joined forces to develop “clean” diesel, as part of their effort to meet stringent new regulations on tailpipe emissions being put into place in Europe and the United States. By 2011 Volkswagen had built a growing following for its diesel Jetta, Beetle and other models, and other automakers raced to offer their own clean diesel models.

Fiat Chrysler introduced diesel versions of its Ram 1500 pickup and Jeep Grand Cherokee in 2014.

For a time, environmental scientists marveled at the vehicles, which offered power and efficiency for moderate prices while seeming to emit far less pollution than older diesel engines. But research at West Virginia University eventually showed that while Volkswagen’s diesels passed emissions tests, they released far more pollutants than allowed in real-world driving.

In 2015, Volkswagen acknowledged that its diesel cars were equipped with software that activated robust emissions controls in testing but disabled them at other times, a revelation whose legal and reputational costs continue to reverberate. After that, the United States Environmental Protection Agency began looking more closely at other companies that made diesel vehicles, and Fiat Chrysler was one of them.

In 2017, the Justice Department sued the company over the E.P.A.’s finding that Fiat Chrysler had used illegal engine-control software that turned off pollution controls under certain driving conditions. The E.P.A. contended that the software enabled the vehicles to pass emissions tests while allowing them to release higher levels of pollutants in normal driving, conduct the agency called “serious and egregious.”