AUBURN HILLS, MI – Fiat Chrysler – the company behind Dodge and Ram trucks – is the latest manufacturer to be accused of tampering with emissions controls.

The U.S. Environmental Protection Agency (EPA) says the company violated the Clean Air Act by installing and failing to disclose engine management software in light-duty Model Year 2014, 2015 and 2016 Jeep Grand Cherokees and Dodge Ram 1500 trucks with three-liter diesel engines. The eight undisclosed “pieces of software” can alter emissions such as smog-producing NOx, the EPA says.

Fiat Chrysler responded in a statement that it is “disappointed” in the notice of violation, and stressed that its diesel-powered vehicles meet “all applicable regulatory requirements”. The company’s engines include emissions-reducing technologies such as Selective Catalytic Reduction. Stock prices plunged to $9.05 per share, from a previous close of $11.09, on the news.

Investigations have been launched by both the EPA and California Air Resources Board (CARB).

The announcement comes on the heels of Volkswagen agreeing to pay US $4.3 billion for its own emissions scandal, as well as pleading guilty to conspiracy, obstruction of justice, and importing vehicles by using false statements. Six senior Volkswagen employees were also indicted in that case this week, while prosecutors in the U.S. Justice Department claim that at least 40 company employees have been involved in destroying related evidence.

Nearly 600,000 Volkswagen, Audi and Porsche models in the U.S. had software that allowed the vehicles to pass government tests, but switch off when driving over the road. The changes were introduced as early as 2006, when it became clear that new engines would not meet 2007 emission standards, according to a plea agreement.

“All automakers must play by the same rules, and we will continue to hold companies accountable that gain an unfair and illegal competitive advantage,” said Cynthia Giles, assistant administrator for EPA’s Office of Enforcement and Compliance Assurance.

“Once again, a major automaker made the business decision to skirt the rules and got caught,” said Mary D. Nichols, chairwoman of CARB. “CARB and U.S. EPA made a commitment to enhanced testing as the Volkswagen case developed, and this is a result of that collaboration.”

The EPA expanded the search for defeat devices on light-duty vehicles in September 2015.