U.S. environmental regulators say that software allowing six-cylinder Volkswagen diesel engines to cheat on pollution tests is on more models than originally thought.

The Environmental Protection Agency on Nov. 2 accused VW of putting the software on about 10,000 Volkswagen and Audi vehicles with 3-liter engines from the 2014 through 2016 model years. But on Thursday, the German automaker told the EPA and California Air Resources Board that the same software is on vehicles going back to the 2009 model year.

The EPA says in a statement that it will investigate and take appropriate action.

Volkswagen has denied that the software allows the cars to cheat on emissions tests.