(Diego M. Radzinschi)

Documents seized during a March raid on the Munich office of Jones Day cannot be used by German prosecutors investigating the Volkswagen emissions scandal, a German high court has ruled.

Authorities raided the office of the firm, legal adviser to VW, in March—a move that VW called a “clear violation of legal principles.”

The automaker saw a previous attempt to stop prosecutors from using the seized material dismissed by a Munich district court in April. That decision was overturned on Wednesday by Germany’s Federal Constitutional Court—the country’s highest court—which ruled that prosecutors are not permitted to analyze the law firm’s documents.

Jones Day was hired by VW to oversee an internal investigation after it was learned that the German carmaker had been using secretive software to cheat emissions tests.

VW reached a $15.3 billion settlement last year to resolve consumer class actions relating to the scandal, and in February agreed to pay another $1.2 billion to settle additional claims and a suit brought by the Federal Trade Commission. Earlier this year, the auto giant agreed to pay $4.3 billion in civil and criminal penalties in an agreement with the U.S. Department of Justice.

Jones Day’s probe has not been publicly released, but it reportedly has found instances of wrongdoing by certain VW executives, while exonerating members of the company’s management board.

Shortly after its offices were raided, Jones Day retained Frankfurt criminal defense lawyer Jürgen Klengel, a former equity partner at White & Case who started his own firm in February, according to Juve.

Jones Day did not immediately respond to a request for comment.