The Illinois attorney general's office has issued a subpoena to Volkswagen over its manipulation of emissions tests.
The German automaker has admitted to rigging cars to pass emissions tests, while in actual driving conditions, the vehicles will emit pollutants up to 40 times above what is allowed in the U.S.
Attorney General Lisa Madigan says Illinois is one of many states investigating the company.
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"We have to fashion both what happened and then how do we figure out a resolution for the people who own these cars, whose cars are now owned a lot less now than when they purchased them," Madigan said.
Madigan says her office has dealt with car-related cases before, but never with emissions violations on this scale.
Volkswagen may eventually face billions of dollars in fines for violating the Clean Air Act. The so-called "defeat device" which allowed the cars to pass emissions tests was included in nearly 500,000 cars sold in the U.S.