BERLIN, May 3 (Xinhua) -- Four witnesses are to testify in court that former Volkswagen chief executive officer (CEO) Martin Winterkorn already knew about exhaust gas manipulations before U.S. authorities uncovered them, German media reported on Friday.

According to research by the German newspaper Sueddeutsche Zeitung, NDR and WDR, the Brunswick public prosecutor's office has summoned four witnesses to convict the 71-year-old ex-manager of "serious fraud, embezzlement, and unfair competition".

All of the four had been high-ranking VW employees at the time of the scandal and were themselves on the list of accused persons in the diesel fraud case, German media reported.

The witnesses who had already testified against Winterkorn will allegedly repeat this in a German court and testify that the ex-Volkswagen CEO knew about the manipulations before they were uncovered on September 18, 2015.

The witnesses reportedly agreed on a central accusation of the public prosecutor's office in Brunswick, namely that Winterkorn was explicitly informed of the fraud software at a special Volkswagen roundtable on July 27, 2015.

The former Volkswagen CEO has insisted that he was only informed of exhaust gas manipulations in September 2015.

The German public radio and television broadcaster NDR noted that whether Winterkorn knew about the fraud software as early as 2014, as the Brunswick public prosecutor's office claimed, would be "the subject of a complicated taking of evidence in the trial".

Despite extensive investigations, the German public prosecutors had not found "any significant paper trail from earlier times," the German broadcaster reported.

The fact that the witnesses are themselves accused in the exhaust manipulation scandal could also be a "problem for the prosecution," the NDR reported.

According to German media reports, several participants in the ongoing legal proceedings against the former Volkswagen CEO have criticized the Brunswick public prosecutor's office.

The investigators had supposedly "ignored the exonerating aspects and had not considered contradictions" in the accusations made against Winterkorn and other accused persons.

The public prosecutor's office in Brunswick had officially brought charges against the former Volkswagen CEO on April 15, who was accused of a "particularly serious case of fraud".

According to the Brunswick regional court, however, several months will pass before a possible court case can begin. The almost 700-page indictment of the public prosecutor's office will have to be examined.

Following investigations by U.S. authorities, Volkswagen admitted in September 2015 that it had used illegal software in around eleven million diesel vehicles worldwide. This software depressed pollutant emissions during emission tests so that they appeared lower.

Currently, more than 60,000 individual proceedings are underway in Germany against Volkswagen or its subsidiaries in connection with the exhaust scandal.

According to the Volkswagen, the German car giant has allocated reserves of around 30 billion euros (33.4 billion U.S. dollars) for the legal cost of the diesel scandal by now.