NEW YORK — The Environmental Protection Agency on Thursday accused Fiat Chrysler of using secret software that allowed illegal excess emissions from at least 104,000 diesel vehicles.

Affected models include 2014-2016 Jeep Grand Cherokees and Dodge Ram 1500 trucks with 3-liter diesel engines that were sold in the United States, the agency said.


The software resulted in excess emissions of nitrogen oxides, which have harmful health effects, from the vehicles, the agency said.

The excess in emissions “threatens public health by polluting the air we breathe,” said Cynthia Giles, an assistant administrator at the EPA.

Giles stopped short of calling the software “defeat devices,” which Volkswagen used to cheat on diesel emissions tests. Volkswagen is now engulfed in a scandal over the devices.

But she said there was no doubt that Fiat Chrysler’s software “is contributing to illegal pollution.”

Giles’s description of the software showed great similarities with Volkswagen’s defeat device. It was designed so that during emissions tests, the cars met legal standards. But EPA investigations showed that under normal driving conditions, the software reduced the effectiveness of emissions controls, Giles said.

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Fiat Chrysler has been notified that the cars violate the Clean Air Act. The automaker has not yet given the EPA or the California Air Resources Board, which jointly investigated the vehicles, a satisfactory answer on the accusations, Giles said.

In a statement, Fiat Chrysler said it “believes that its emission control systems meet the applicable requirements.”

The company added that it “intends to work with the incoming administration to present its case and resolve this matter fairly and equitably” and assure the agency and customers that it meets regulatory requirements.

The discovery of Volkswagen’s cheating in its vehicles, 600,000 of which were sold in the United States, has set off numerous investigations.

On Wednesday, federal prosecutors announced criminal charges against six Volkswagen executives for their roles in the company’s emissions-cheating scandal. Volkswagen also formally pleaded guilty to charges of conspiracy to commit wire fraud and to violate the Clean Air Act, customs violations, and obstruction of justice.

The financial cost to Volkswagen has been hefty. The German automaker is set to pay $4.3 billion in criminal and civil penalties in connection with the federal investigation, bringing the total cost of the deception to Volkswagen in the United States, including settlements of suits by car owners, to $20 billion — one of the costliest corporate scandals in history.

The scandal has also led US regulators to test a wide range of vehicles for compliance with federal emissions standards. Giles declined to identify which automakers’ cars were being investigated but said investigations were continuing.

Stock in Fiat Chrysler tumbled after the EPA’s announcement, falling more than 15 percent before heavy volumes forced the New York Stock Exchange to suspend trading.

The accusations come at a difficult time for Fiat Chrysler.

Until recently, Fiat Chrysler enjoyed rapidly rising sales in the US market, outpacing many of its rivals such as Ford and General Motors. Americans’ increasing interest in trucks and sport utility vehicles drove brisk sales of Jeeps and Ram pickups, even as Fiat Chrysler’s car sales languished.

But last year a lawsuit accused the company of inflating sales figures, and it came under investigation by the Securities and Exchange Commission. The investigation has not yet been completed. After news of the investigation broke, Fiat Chrysler’s sales slowed. Last year, the company sold 2.24 million cars and light trucks, a decline of 0.4 percent from 2015.

Environmental advocates used the accusations against Chrysler to highlight the role of the EPA and to issue warnings about President-elect Donald Trump’s expected plans to reduce the agency’s scope and authority.

“It’s very important that they’re doing this now,” said Frank O’Donnell, the president of Clean Air Watch, a Washington advocacy group. “They’ve got polluter lobbyists massing at the gate of both Congress and the White House. This case scores the importance of keeping a federal environmental cop on the beat at EPA.”

Trump has vowed to “take a tremendous amount out” of the EPA. His nominee to head the agency, Scott Pruitt, the attorney general of Oklahoma, has led a legal charge to dismantle many of the agency’s signature clean air regulations, and he has pushed broadly to diminish federal oversight of the environment, sending authority from Washington to the states.