The recent Volkswagen Group emissions scandal has so far focused on the company’s 2.0-liter diesel engines, but may soon broaden to encompass more engines, and more vehicles.

The Environmental Protection Agency (EPA) and the California Resource Air Board (CARB) separately announced plans to expand testing to Volkswagen’s 3.0-liter engines. Models offering this 6-cylinder engine include the Porsche Cayenne, Volkswagen Toureg and Audi A6.

To date, Volkswagen has only acknowledged that its emission defeat devices were installed in its 2.0-liter engines. The number of vehicles affected is estimated to be 11 million worldwide and includes models such as the Audi A3 and Volkswagen’s Golf, Jetta and Passat.

SEE ALSO: What Volkswagen’s ‘Dieselgate’ Is, and Why it Matters

CARB held numerous meetings with the carmaker to discuss emissions discrepancies, said Communications Direction Stanley Young, but it was only recently that Volkswagen executives admitted using a defeat device.

“They literally ran out of excuses,” Young said. That first “investigation looked at two-liter four-cylinder engines. Now we’re going to start looking at six-cylinder, three-liter diesel engines.”

Regulatory agencies in the U.S. aren’t the only ones scrutinizing Volkswagen’s diesel engines, either. Diesels are prevalent throughout Europe, partially because the price of diesel fuel is cheaper than gasoline. Out of the 11 million vehicles affected by the scandal, analysts estimate European markets account for 10 million.

Here, “Volkswagen is the dominant manufacturer, with more than double the market share of any competitor,” reported The New York Times, “and diesels account for more than half of all vehicles sold.”

SEE ALSO: What Volkswagen is Doing to Help its Dealers In Wake of Halted Diesel Sales

In the aftermath of Volkswagen’s admission, the European Union also announced that it will be tightening its standards.

“We need to get to the bottom of this,” European Commission representative Lucia Caudet said. “For the sake of our consumers and the environment, we need certainty that industry scrupulously respects emissions limits.”

Even if the investigation does implicate other Volkswagen engines, the European Automobile Manufacturers Association said it hasn’t seen any evidence indicating that defeat devices are an industry-wide problem.