Auto supplier Bosch has been accused of hiding the use of Volkswagen's secret "defeat devices" which it helped design.
It comes after earlier claims VW car owners that Bosch was a "knowing and active participant" in the carmaker's decade-long scheme to evade US anti-pollution laws.
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According to a fresh court filing by VW owners' in the US, Bosch "did not disclose its knowledge of the illegal defeat device in any ... communications with U.S. regulators". The lawsuit contains some allegations which were redacted last month.
It pointed to a June 2008 email from Bosch to VW which demanded: "Volkswagen indemnify Bosch for any legal exposure arising from work on the defeat device".
It also accuses Bosch chief executive, Volkmar Denner, of knowing about the use of the defeat device.
In June, VW reached a settlement in the US to pay roughly $14.7bn (£11.2bn) to compensate vehicle owners and fund government programmes to boost emissions technology and mitigate air pollution. However, private complaints are still going through the courts.
Last year, the car manufacturer confessed to fitting vehicles with devices designed to cheat emissions tests, sparking a slew of investigations and legal actions in multiple countries. More than 11 million vehicles worldwide have been affected.
Bosch has not been charged with any wrongdoing.