Four Audi AG officials have been indicted for their roles in the German automaker's effort to rig hundreds of thousands of diesel cars to cheat U.S. emission standards.

Richard Bauder, Axel Eisner, Stefan Knirsch and Carsten Nagel, all German residents, have been indicted and charged with conspiracy to defraud the United States, to commit wire fraud, and to violate the Clean Air Act. The indictments by a federal grand jury in Detroit's U.S. District Court for the Eastern District of Michigan were released Thursday.

Each charge is punishable by up to 20 years in federal prison.

Audi and its parent company, Volkswagen AG, have been under fire in the United States since being accused by the U.S. Environmental Protection Agency in September 2015 of selling diesels for years with software that activated required air pollution equipment only during emissions tests. They had been marketed as “clean diesels” for the company’s Volkswagen, Audi and Porsche brands between 2008 and 2015.

Volkswagen and Audi have admitted to programming its diesel cars to trick emissions testers into believing the engines released far less pollution into the air than they actually do, in violation of the federal Clean Air Act. Regulators have said that in normal driving they emitted up to 40 times more smog-causing nitrogen oxide than the legal limit.

Volkswagen and Audi have stopped selling diesel cars in the U.S. since it admitted to the scheme.

Audi did not immediately respond to a request for comment from The Detroit News.

According to the indictments that were filed Thursday, Bauder was the head of Audi’s Diesel Engine Development Department in Neckarsulm, Germany. Eiser was head of Audi’s Engine Development Division in Ingolstadt, Germany.

Knirsch worked as head of Audi’s Engine Development Division in Ingolstadt, Germany, and Nagel served as the head of Engine Registration within Audi’s Registration and Testing Department in Neckarsulm, Germany.

The quartet joins six present and former Volkswagen AG executives who were charged in 2017 along with the company with three criminal felony counts for what regulators called a “10-year conspiracy” to rig hundreds of thousands of diesel cars to cheat U.S. emission standards. Volkswagen is also being forced to pay $2.8 billion in criminal fines and $1.5 billion in civil penalties related to the fraud.

The indictments against Audi officials are the latest in a string of decisive moves by the federal government to stamp out emission cheating. In addition to the Volkswagen case, Fiat Chrysler Automobiles NV agreed to an $800 million settlement with the Justice Department and EPA over alleged diesel cheating.

klaing@detroitnews.com

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Twitter: @Keith_Laing