The federal government has sued Fiat Chrysler Automobiles for using illegal engine-control software in its diesel vehicles so that they would perform differently during emissions testing than under normal driving conditions.

The Department of Justice filed the complaint on May 23, claiming FCA installed the software in 104,000 diesel Ram 1500 and Jeep Grand Cherokee vehicles. The filing comes days after FCA submitted a proposed software modification that it hoped would resolve the issue. FCA said it was disappointed about the filing.

"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," according to an FCA statement.

FCA hasn't been able to sell its diesel models since January, when the U.S. Environmental Protection Agency and California Air Resources Board issued notices of violation that the 3.0L V-6 powering the vehicles was emitting increased levels of nitrogen oxides that violate the Clean Air Act.

FCA has been in discussions with officials from the U.S. Department of Justice's Environment and Natural Resources Division.

The complaint cites eight "software-based features" that lessen the effectiveness of the emissions control systems during normal driving.

Read the full complaint here.