The Environmental Protection Agency said Monday that it will screen for so-called "defeat devices" in diesel vehicles on the road that are produced by other manufacturers. The California Air Resources Board is a partner in the investigation, an EPA spokesperson said in an email to AFP.
The EPA declined to identify the automakers it is looking at.
The expanded investigation came in the wake of Friday's announcement by the EPA and the California authorities that Volkswagen, Europe's biggest automaker, had admitted it equipped 482,000 cars in the United States with the illegal defeat devices.
The vehicles affected are four-cylinder VW and Audi diesel cars built since 2008.
Under the US Clean Air Act, fines of up to $37,500 may be imposed on each car, meaning VW could be fined more than $18 billion.