Discovery of such devices in Volkswagen’s “clean diesel” cars, has weakenedthe world’s largest automaker, slashed its market value, raised the specter of criminal investigations and fines and forced its CEO to resign.

California air regulators had a hand in uncovering Volkswagen’s fraud. And on Friday, the California Air Resources Board sent other automakers a letter saying it will now subject other diesel cars to the same tests that found Volkswagen’s defeat devices. It also reminded car companies that such devices are illegal.

The board said it “will take actions that protect public health, air quality and consumers by aggressively continuing its ongoing investigation into Volkswagen’s defeat device, with additional testing and investigation of the potential presence of similar devices in vehicles across manufacturers.”

Last week, the board and the U.S. Environmental Protection Agency reported that Volkswagen had installed defeat device software in diesel cars from 2009 through 2015. The software turns on all of the vehicle’s air-pollution controls only when it senses that an emissions test is under way.

The False Claims Act allows private citizens to report fraud on the Government and to collect a percentage of moneys the Government obtains if the report is verified.

Jeffrey Newman represents whistleblowers