FRANKFURT, Germany — German prosecutors said Tuesday that they had expanded their investigation into the illegal manipulation of tailpipe emissions by Volkswagen, raising the number of suspects from six to 17.

In line with German privacy rules, none of the suspects’ names were disclosed.

So far, no suspects are current or former members of the company’s management board, said Klaus Ziehe, spokesman for the state’s attorney’s office in Braunschweig, a city near Volkswagen headquarters in Wolfsburg.

But Ziehe said the investigation was continuing, and that prosecutors had not ruled out involvement by top management. “We are looking at all levels, including the management board level,’’ he said.

Ziehe declined to say what kinds of jobs the suspects held or where in the company they worked.

Volkswagen did not immediately respond to a request for comment.

In a second development, French prosecutors have opened a formal fraud investigation into the Volkswagen diesel cheating, following a preliminary inquiry that began in October. Nathalie Homobono, the head of the French anti-fraud authority, said at a news conference on Monday that a preliminary investigation had found that Volkswagen’s fraud was “intentional.’’

The increasing number of suspects, none of whom have been formally charged, raises the stakes for Volkswagen as the company takes a more aggressive stance against accusations that knowledge of the wrongdoing was widespread in the company.

In a court filing last week, Volkswagen said that, until last summer, no members of its management board were aware that software in 11 million diesel vehicles had been programmed to cheat on emissions tests. The filing portrayed the software manipulation as the work of “individual employees’’ whose activities could not be detected by outsiders.

“The management board of Volkswagen had no knowledge either of the programming of the impermissible software nor of its later use in affected diesel motors,’’ the company said in a document filed on Feb. 29 as part of its defense against lawsuits by shareholders.

The filing was not public, though Volkswagen released a summary last week. The New York Times subsequently acquired a full copy of the document, which was more than 100 pages.

The shareholders accuse Volkswagen of violating German laws requiring company managers to disclose information that could affect the share price, so the dates when top managers learned of the wrongdoing are crucial to the court case.

The illegal software recognized when a car was being tested and turned up pollution controls.

At other times, the cars emitted many times the allowed amount of nitrogen oxides, which are linked to serious lung ailments and premature deaths.

Ziehe of the Braunschweig prosecutor’s office said on Tuesday that investigators were looking into people who had been aware of the wrongdoing and took no action, as well as people responsible for the illegal programming.