Volkswagen AG is facing a new round of questions as the U.S. Environmental Protection Agency says it has expanded its investigation of an emissions cheating scandal to include more diesel models, including Porsche.

The EPA has determined certain 2014-2016 VW, Audi and Porsche models with 3.0 liter engines have equipment to turn off emissions systems when they're not being tested, the agency said in a letter posted to its website.

The EPA and the California Air Resources Board announced Sept. 18 that they were investigating Volkswagen after the car maker admitted to rigging the emissions systems of diesel vehicles so they would pass pollution tests.

The revelation rocked the world's second-largest automaker, leading to the resignation of its chief executive, Martin Winterkorn, and an announced charge of 6.5 billion euros ($7.3 billion) for recall repair costs. Under the Clean Air Act, Volkswagen may be liable for fines as high as $18 billion, based on maximum penalties per car involved.

Volkswagen AG's top U.S. executive told lawmakers in Congress last month the emissions cheating was carried out by a few engineers in Germany without any formal decision by the company. He conceded it may take years to retrofit the bulk of the affected vehicles.

Michael Horn, president and chief executive officer of Volkswagen of America, said most of the cars will need more than a software update to comply with U.S. requirements. Volkswagen is looking at adding hardware such as improved catalytic converters or urea tanks, he said.

Volkswagen will compensate customers and address any impact on performance, Horn said. He said the fixes will maintain the car's fuel efficiency.

Volkswagen AG is facing a new round of questions as the U.S. Environmental Protection Agency says it has expanded its investigation of an emissions cheating scandal to include more diesel models, including Porsche.

The EPA has determined certain 2014-2016 VW, Audi and Porsche models with 3.0 liter engines have equipment to turn off emissions systems when they're not being tested, the agency said in a letter posted to its website.

The EPA and the California Air Resources Board announced Sept. 18 that they were investigating Volkswagen after the car maker admitted to rigging the emissions systems of diesel vehicles so they would pass pollution tests.

The revelation rocked the world's second-largest automaker, leading to the resignation of its chief executive, Martin Winterkorn, and an announced charge of 6.5 billion euros ($7.3 billion) for recall repair costs. Under the Clean Air Act, Volkswagen may be liable for fines as high as $18 billion, based on maximum penalties per car involved.

Volkswagen AG's top U.S. executive told lawmakers in Congress last month the emissions cheating was carried out by a few engineers in Germany without any formal decision by the company. He conceded it may take years to retrofit the bulk of the affected vehicles.

Michael Horn, president and chief executive officer of Volkswagen of America, said most of the cars will need more than a software update to comply with U.S. requirements. Volkswagen is looking at adding hardware such as improved catalytic converters or urea tanks, he said.

Volkswagen will compensate customers and address any impact on performance, Horn said. He said the fixes will maintain the car's fuel efficiency.