Fiat Chrysler has been accused of not telling authorities about software that regulates emissions in thousands of its diesel vehicles.
The US Environmental Protection Agency (EPA) said the company broke the law by installing, but failing to disclose, the technology in more than 100,000 Jeeps and Dodge vehicles.
The agency said the software caused higher nitrogen oxide levels.
Fiat Chrysler boss Sergio Marchionne said the firm had done nothing illegal.
"There was never any intent of creating conditions that were designed to defeat the testing process. This is absolute nonsense," he added.
Shares in Fiat Chrysler ended the day 10.3% lower in New York and closed 16% lower in Milan.
The EPA said Fiat Chrysler could be liable for fines of about $44,500 per vehicle, which would mean a total of about $4.6bn (£3.8bn).
According to the EPA, the 104,000 vehicles affected were 2014, 2015 and 2016 model years of the Jeep Grand Cherokee SUV and Dodge Ram truck.
Between "one-sixth and one-seventh" of the vehicles were sold in Europe, with the rest in the US, according to Fiat Chrysler.
In a press conference, the EPA said at least eight types of emissions control software on the vehicles had not been disclosed to the agency.
It also suggested some of the software appeared to make cars perform differently during emissions tests.
"Failing to disclose software that affects emissions in a vehicle's engine is a serious violation of the law, which can result in harmful pollution in the air we breathe," said EPA official Cynthia Giles. "All automakers must play by the same rules."
Fiat Chrysler said it would demonstrate that its emissions control strategies were justified and thus were not "defeat devices" designed to get around emissions tests.
It also said it had responded to requests for information from US authorities and sought to explain its position.
On Wednesday, Volkswagen pleaded guilty to three criminal charges to settle charges over its emissions-rigging scandal.
The Department of Justice said VW had a long-running scheme to sell about 590,000 diesel vehicles in the US fitted with a defeat device to cheat on emissions tests.
The firm will pay $2.8bn in criminal fines and $1.5bn in civil penalties.
The EPA is reviewing other carmakers' emissions systems, but it is unclear if it has found any additional wrongdoing.
In April, Mercedes Benz owner Daimler said the US Justice Department had asked the firm to investigate its emissions certification processes.