The raids came as Volkswagen's U.S. chief Michael Horn faced a grilling before Congress, where he sought to distance himself from the scandal while blaming it on engineers in Germany.

Horn told a committee that he had learned in early 2014 that the group's ostensibly environmentally friendly diesel cars breached pollution rules.

But he said he did not know until last month that "defeat devices" had been installed deliberately in the vehicles to help them cheat U.S. pollution tests.

When the emissions problem was first discovered by U.S. university researchers last year, he said, "I had no understanding what a defeat device was. And I had no indication whatsoever that a defeat device could have been in our cars."

The world's largest automaker sank into the deepest crisis of its history after revealing last month that it equipped 11 million diesel VWs and Audis with software that switches the engine to a low-emissions mode during tests.

The software then turns off pollution controls when the vehicle is on the road, allowing it to spew out harmful levels of toxic gases. (AFP)