Former Volkswagen chief executive Martin Winterkorn isn’t expected to testify in a trial against Porsche SE brought forward by investors. Bosch chief executive Volkmar Denner won’t testify either.

Speaking at the opening of the trial in a Stuttgart regional court, Judge Fabian Richter Reuschle said most of the two dozen witnesses called in the case have refused to give evidence, Automotive News Europe reports.

“We have to accept that we cannot force them to incriminate themselves,” Richter Reuschle said.

Shareholders are seeking compensation from holding group Porsche SE, claiming that they were not informed swiftly enough about the liabilities the German automaker could face over its use of software designed to cheat emissions testing in the United States. A separate suit with the same claims was filed in Brunswick, Germany a few days ago.

It is possible that the Stuttgart trial will be suspended to await the outcome of the Brunswick trial.

Although the German car manufacturer has admitted to cheating diesel emissions tests, it denies assertions that it didn’t tell shareholders fast enough. Plaintiffs argue that if they knew of the scope of the potential liabilities, they could have sold shares or not made purchases much earlier than they did.

Many of the key components of VW’s diesel engines were supplied by Bosch, which is why the latter’s chief executive, Volkmar Denner, was called to testify. However, the automotive parts supplier says that it was the responsibility of Volkswagen to integrate the parts into its powertrains.

Martin Winterkorn resigned as Volkswagen chief executive just days after the diesel scandal was made public.