Authorities in the US are now turning their attention to the German auto supplier Robert Bosch GmbH over its role in the Volkswagen emissions scandal.

Federal prosecutors with the Department of Justice are examining if Bosch knew or participated in Volkswagen's efforts to fiddle diesel emissions tests. They are also investigating how deeply the scheme permeated VW's hierarchy, according to people familiar with the matter. Reports suggest that the investigation is at an early stage and there is no indication that they have found any evidence of wrongdoing at Bosch.

Bosch provides the engine control module, called EDC17, and basic software for nearly all the four-cylinder diesel cars sold in North America, including by Volkswagen, BMW AG and Daimler AG's Mercedes-Benz. Those systems regulate how a vehicle cleans burned-up fuel before it is expelled as exhaust.

Volkswagen had the engine software modified to turn on the vehicle's emission control system when it was being tested in the lab, then turn it off when the vehicle was on the road, according to U.S. regulators.

For authorities to bring charges against Bosch, they would have to prove the supplier knew that their technology was being used by Volkswagen to evade emissions requirements. But it is not clear what role Bosch played, how closely it worked with VW to modify the engine management software and how much it knew about VW's intentions to use software to cheat on emissions standards.