The US Justice Department has reportedly opened a criminal investigation into Volkswagen over the German automaker's alleged cheating on US emissions tests. This follows an Environmental Protection Agency notice issued Friday claiming Volkswagen deliberately dodged air-pollution rules so that nearly 500,000 diesel-powered automobiles sold since 2008 would appear to meet federal standards. The EPA issued a violation of the Clean Air Act and opened its own investigation. Now, the Justice Department’s Environment and Natural Resources Division is conducting a separate probe alongside mounting pressure from the German government, according to The Wall Street Journal today.

The allegations cover popular Volkswagen models including the Jetta, Beetle, Audi A3, Golf, and Passat. The EPA says these vehicles were equipped with a software algorithm, called a defeat device, that detects when an emissions test is being conducted and turns on full emissions controls only throughout the duration of the test. After the test is over, the car resumes emitting nitrogen oxides at up to 40 times the standard regulatory limit. "By making and selling vehicles with defeat devices that allowed for higher levels of air emissions than were certified to EPA, Volkswagen violated two important provisions of the Clean Air Act," the EPA said in a press release on Friday.

Volkswagen CEO Martin Winterkorn says the company is cooperating with authorities

The EPA discovered the defeat devices in Volkswagen vehicles after the organization teamed up with the California Air Resources Board and demanded Volkswagen explain an independent analysis conducted by researchers at West Virginia University. Volkswagen then allegedly came clean to the EPA. The Volkswagen vehicles do not pose an immediate safety hazard, the EPA says, and are safe to drive and resell.

Following the news, the Oversight and Investigations Subcommittee has announced a hearing to address the allegations in the coming weeks, to be held by House Energy and Commerce Committee chairman Fred Upton and Oversight and Investigations Subcommittee chairman Tim Murphy. "We will follow the facts. We are also concerned that auto consumers may have been deceived — that what they were purchasing did not come as advertised," Upton and Murphy wrote today in a joint statement. "The American people deserve answers and assurances that this will not happen again. We intend to get those answers."

Volkswagen’s troubles could get even worse: The Detroit News is reporting that a diesel engine used by Audi and Porsche is also being investigated by the EPA, bringing more affected vehicles into the fold.