FRANKFURT — The investigation into emissions fraud at Volkswagen reached the very top of the company on Sunday after the carmaker said that the chairman of the supervisory board, Hans Dieter Pötsch, is suspected by German prosecutors of violating securities laws.
Mr. Pötsch, the former chief financial officer at Volkswagen, is suspected of failing to notify shareholders quickly enough of the financial risks of the diesel emissions cheating scandal, which has already led to a $15 billion settlement in the United States and caused the stock price to plunge.
The disclosure that Mr. Pötsch is the subject of an investigation is likely to intensify criticism that Volkswagen remains in the hands of many of the longtime insiders who were in charge while the company was producing millions of cars that were deliberately designed to cheat on air-quality tests. More than a year after the company was accused of wrongdoing, the scandal is still widening and the damage to Volkswagen’s finances and reputation continues to expand.
The investigation of Mr. Pötsch could also provide ammunition to investor groups and mutual funds that are suing Volkswagen in the United States and Germany. The suits claim that Volkswagen managers were aware of the impending scandal and failed to notify shareholders as required by law. The suits could cost the company additional billions of euros.
A confidant of the Porsche and Piëch families, who own a majority of Volkswagen’s voting shares, Mr. Pötsch was elevated to chairman of the supervisory board in October 2015. That was a few weeks after the Environmental Protection Agency accused the carmaker of manipulating engine software to conceal illegally high levels of nitrogen oxide emissions.
Mr. Pötsch had been the chief financial officer of Volkswagen since 2003 and a member of the company’s management board. As chairman of the supervisory board, Mr. Pötsch oversees the management board.
In a statement, Volkswagen said that its management board “duly fulfilled its disclosure obligation under German capital markets law.”
Volkswagen is also under investigation in the United States, not only for programming cars to cheat but also for orchestrating an elaborate cover-up starting in early 2014 after tests first cast doubt on what the company claimed were “clean diesel” cars.
In fact, the Volkswagen cars emitted as much as 40 times the permitted levels of nitrogen oxides, a family of gases that can cause health problems including asthma and cancer. Nitrogen oxides also contribute to global warming and acid rain, and are a leading cause of the smog that chokes cities like Los Angeles.
Volkswagen engineers went so far as to concoct fake engineering data to try to explain a huge discrepancy between the readings in official laboratories and how much the cars polluted on the road, said Alberto Ayala, deputy executive officer of the California Air Resources Board, which did much of the detective work that led to Volkswagen’s exposure.
“They lied through their teeth,” Mr. Ayala said in an interview in California last month.
The cover-up, which lasted more than a year, ultimately raised the cost of the scandal to Volkswagen. It has not been able to take advantage of lower financial penalties normally available to corporate wrongdoers who are forthcoming with information and who swiftly take disciplinary action against the responsible employees.
Volkswagen has portrayed the malfeasance as the work of midlevel engineers and managers acting without knowledge of top management. But that position has become increasingly difficult to defend as more information becomes available from court documents. Lawsuits against Volkswagen by car owners as well as state attorneys general portray a vast conspiracy involving hundreds of Volkswagen employees as well as suppliers like Robert Bosch, the German company that manufactured engine computers for affected vehicles in the United States.
The identification of Mr. Pötsch as a target of the investigation could also intensify criticism of the Porsche and Pïech families, descendants of Volkswagen’s founder, Ferdinand Porsche, who own a majority of Volkswagen’s voting shares. Mr. Pötsch is closely associated with the family and is also chief executive of Porsche automobile Holding SE, the holding company for the family’s shares in Volkswagen. Other investors have criticized the families for poor oversight of Volkswagen and helping to create the corporate culture that led to the wrongdoing.
Just one person has been formally charged in the case. In September, James Liang, a Volkswagen engineer, pleaded guilty to federal charges of conspiring to defraud regulators and car owners. Mr. Liang agreed to cooperate with investigators and has not yet been sentenced.
Mr. Liang was in the United States, but most of the other potential suspects are in Germany. Attempts to work out plea agreements with the others have so far foundered on differences between German and American laws. In Germany, prosecutors have much less scope to offer defendants reduced sentences in return for guilty pleas.
Many of the potential suspects are being careful not to leave Germany, lest they be arrested on American warrants, according to lawyers as well as engineers who have been questioned by investigators. Germany does not usually extradite its own citizens, but there is no protection for Germans traveling in other European countries.
Prosecutors and Volkswagen have previously disclosed that Martin Winterkorn, the former chief executive of Volkswagen, and Herbert Diess, a member of the management board responsible for the Volkswagen brand, are also under investigation for violating the company’s duty to disclose information that could affect the company’s share price.
Mr. Winterkorn resigned shortly after the E.PA. first accused Volkswagen of wrongdoing in September 2015, but Mr. Diess remains a member of the management board.
Volkswagen shares have lost a quarter of their value since the scandal came to light.
The state of Lower Saxony, which owns 20 percent of Volkswagen shares and occupies two seats on the supervisory board, said on Sunday that Mr. Pötsch was innocent until proven guilty and that it would be wrong to arrive at “rash conclusions.”
In a statement, the state said that members of the supervisory board were briefed on Friday on the latest findings by internal investigators. The report provided “no occasion for further measures,” the state said.