VW supplier Robert Bosch GmbH is named in a recent court filing regarding the hiding of the automaker's diesel problem.

Volkswagen AG has already either paid out or is on the hook for more than $15.3 billion in fines and penalties related to its diesel emissions scandal.

Now one of the company’s top suppliers, Robert Bosch GmbH, is facing charges that it helped cover up VW’s cheating on emission tests, according to a lawyer for U.S. owners of VW vehicles equipped with the affected diesel engines.

In a filing late Tuesday in U.S. District Court in San Francisco, lawyers cited confidential documents turned over by the German automaker to plaintiffs attorneys in making the new allegations against the auto supplier.

Volkswagen has declined to comment on the filing, except to say that it had no effect on its multibillion-dollar settlement of a civil complaint about the diesel scandal. The filing was made a day after sources briefed on the matter said the automaker has held preliminary talks with the U.S. Justice Department to settle a criminal probe into the emissions cheating case.

(Justice Department readying charges against Volkswagen. Click Here for details.)

Most of the allegations involving Bosch remain under seal because the documents have been designated confidential by VW, the plaintiffs’ lawyers said in the court filing.

A Bosch spokeswoman said the company took the allegations seriously and is cooperating in several investigations, but declined to comment further.

(Italian government latest to fine Volkswagen over diesels. For more, Click Here.)

The documents include records and communications between Bosch, VW and U.S. regulators. One 2011 email to the California Air Resources Board, among other communications, demonstrates “Bosch’s deep understanding of what regulators allowed and would not allow, and what Bosch did to help VW obtain approval,” the filing said.

“Bosch played a crucial role in the fraudulent enterprise and profited handsomely from it,” the court papers said.

(EPA discovers three software programs designed to rig 3-liter diesel tests. Click Here for the latest.)

Bosch has not been charged with any wrongdoing. But German prosecutors said in December that they were investigating whether staff at the Stuttgart-based company were involved in the rigging of emissions tests by VW.