German prosecutors have deepened the crisis at Volkswagen by widening an investigation into suspected market manipulation to include the chair of the carmaker’s supervisory board, Hans Dieter Pötsch.

The investigation relates to accusations that VW’s management withheld information about the company’s emissions scandal from shareholders. In September 2015 it had emerged that VW had installed software to deactivate pollution controls on more than 11m diesel vehicles sold worldwide.

Following a complaint by financial watchdog BaFin, prosecutors are already investigating the former VW boss Martin Winterkorn and brand chief Herbert Diess. Pötsch was the Wolfsburg-based carmaker’s finance chief between 2003 and 2015, during which he played a key role in communicating the company’s outlook to investors.

The 65-year-old Austrian moved from the company’s management to its advisory board in October 2015, shortly after the emissions scandal erupted. According to German weekly Der Spiegel, Pötsch’s move earned him a bonus of almost €20m (£17.8m) – a reward that was calculated on the basis of VW’s pre-crisis performance.

If allegations are proven to be true, Pötsch could potentially face a prison sentence of up to 10 years.

Several VW investors have taken legal action to claim compensation for the losses they have made as a result of the emissions scandal. The regional court in Braunschweig has collected 1,400 complaints, pointing to a damages total of about €8bn.

A spokesperson for the carmaker, which employs approximately 600,000 people worldwide, said the company would fully cooperate in the state prosecutor’s investigation.

“Based on a thorough examination by internal and external legal experts, the company reaffirms its belief that VW’s management fulfilled its duties to inform the capital market,” VW said on Sunday.

VW said the company and Pötsch, who became chairman in October 2015, would fully support the prosecutor’s office in its investigation.

VW is controlled by the Porsche and Piëch families through their holding company Porsche SE. Wolfgang Porsche released a statement on Sunday stating that the two families stood with unqualified support behind Pötsch and believed the management had acted in accordance with the law throughout.

Stephan Weil, the state premier of Lower Saxony who shares a seat on the carmaker’s supervisory board with Pötsch, declined to comment “out of respect to the ongoing investigation”.

Adding to the company’s woes on Sunday, German media reported that a US regulator had found another cheat device aimed at lowering carbon dioxide emissions in cars made by VW’s Audi subsidiary.

Bild am Sonntag said the software lowered carbon dioxide emissions by detecting whether a car’s steering wheel was turned as it would be if it was driving on a road. If the steering wheel was not turned, as if it was being tested in a laboratory, the software activated a gear-shifting programme that produced less carbon dioxide, allowing the car to meet the emissions criteria.