German prosecutors searched the offices of a law firm Volkswagen Group hired to conduct an internal investigation into its emissions scandal, drawing the automaker’s condemnation Thursday.

With questions lingering about whether certain top executives were responsible for the scandal, Munich prosecutors foraged through the offices of law firm Jones Day on Wednesday. They also searched German facilities of VW luxury brand Audi.

The Jones Day search was “unacceptable in every respect,” VW said Thursday in a statement.

The automaker called the action a “clear breach of the principles of the rule of law” in Germany and threatened to “take all action at our disposal against these proceedings.”

A Jones Day spokesperson did not immediately respond to a request seeking comment.

German prosecutors have been conducting a criminal investigation targeting dozens of VW workers, including former CEO Martin Winterkorn, who has denied any wrongdoing.

The U.S. Justice Department recently charged six VW workers for their alleged roles in the scandal. And the company agreed to a criminal settlement with U.S. prosecutors, pleading guilty on corporate charges of conspiracy and obstruction of justice. Altogether the scandal has cost the company more than $22 billion so far, including criminal and civil penalties and settlements.

Despite charging six individuals in January, U.S. prosecutors said that their investigation was ongoing and could ensnare additional people.

The company has said that it provided adequate information from Jones Day’s investigation to investigators.

But it has repeatedly refused to provide the investigation to journalists. The company initially said that it needed to withhold the findings to preserve the possibility of a criminal settlement, but has since reversed itself.

Asked by USA TODAY to release the investigation on Tuesday, VW spokesman Michael Brendel said in an email that “there is no Jones Day report” because the Justice Department’s publicly released “statement of facts” incorporates the law firm’s findings.

VW CEO Matthias Mueller pledged Tuesday at the company’s annual meeting to “give the authorities our full and unconditional support.”

He also said Tuesday that the company is continuing to examine the “statement of facts” it agreed to as part of its Justice Department settlement.

“We are currently drawing further conclusions from these findings,” he said.

VW admitted to fitting some 11 million vehicles worldwide with software to cheat emissions tests.

Follow USA TODAY reporter Nathan Bomey on Twitter @NathanBomey.