On Thursday the US Environmental Protection Agency (EPA) announced that Fiat Chrysler (FCA) diesel vehicles were found to have "at least eight" instances of undisclosed software that modified the emissions control systems of the cars. The vehicles implicated in the EPA's Notice of Violation (NOV) include 2014, 2015, and 2016 diesel Jeep Grand Cherokees, as well as Dodge Ram 1500 trucks with 3.0-liter diesel engines. The allegations involve 104,000 vehicles, the EPA said.

The EPA says it's still in talks with FCA and hasn't ordered the company to stop selling affected cars yet, nor is it officially calling the software a "defeat device" just yet until FCA provides a more detailed explanation.

In a press conference, agency officials said that the undisclosed software was discovered after September 2015, when the EPA and the California Air Resources Board (CARB) began doing additional testing on vehicles in the wake of the Volkswagen Group scandal.

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VW Group was discovered to have almost 600,000 diesel vehicles on US roads with some kind of illegal software on them . The software allowed VW Group's cars to pass emissions testing under lab conditions but would reduce the effectiveness of emissions controls under real-world driving conditions, causing the cars to emit nitrogen oxide (NO) far in excess of federal limits.

According to the EPA, FCA's undisclosed software works similarly, too. EPA Assistant Administrator Cynthia Giles told press Thursday morning that the "software is designed such that, during the emissions test, Fiat Chrysler’s cars meet the standards," for NO x emissions. However, the "software reduces the effectiveness of emissions controls when driving at high speed or for long durations," she added.

These kinds of workarounds are not uncommon for car makers to use and are not illegal if they're properly disclosed and approved by the EPA. But efforts to meet emissions standards have driven automakers to install undisclosed devices illegally for decades. In fact in the 1970s, Chrysler—along with GM, Ford, American Motors, Nissan, and Toyota—was reprimanded by the EPA for installing defeat devices in its cars to "defeat the effectiveness of emission control systems under conditions not experienced during EPA’s certification testing." In some instances the defeat devices helped the cars start more easily in cold weather, in others, time-delay switches cut the emissions control systems while the cars shifted from low to high gears.

In Europe, too, rules allow diesel vehicles to cut the emissions control system under certain conditions like cold weather. Automakers have toed a line, though, using emissions control software where "cold" weather means as high as 64 degrees Fahrenheit.

Giles noted during the EPA's press conference that the agency has tested other diesel vehicles since the Volkswagen scandal was made public and found no violations. "It is by no means impossible to make a clean diesel vehicle that meets our standards," she said.

In a statement (PDF), FCA said it would continue to work with the EPA to resolve the issue. "FCA US diesel engines are equipped with state-of-the-art emission control systems hardware, including selective catalytic reduction (SCR). Every auto manufacturer must employ various strategies to control tailpipe emissions in order to balance EPA’s regulatory requirements for low nitrogen oxide (NO x ) emissions and requirements for engine durability and performance, safety and fuel efficiency. FCA US believes that its emission control systems meet the applicable requirements."

FCA added that it had spent "months providing voluminous information" to the EPA and other regulators. The company said it had also made proposals to fix the issues, including "developing extensive software changes to our emissions control strategies that could be implemented in these vehicles immediately to further improve emissions performance."

FCA has not yet been sued, but the EPA says it could be "liable for civil penalties and injunctive relief for the violations alleged in the NOV [Notice of Violation]."

Correction: This story originally said the EPA found the software on the FCA diesels was illegal. In fact, the EPA is still determining whether the software itself was illegal. However, Fiat Chrysler violated EPA rules by not disclosing the software.