The Braunschweig prosecutor's spokesman, Matthias Diekman, said in a statement today that the probe was opened at the behest of Germany's Federal Financial Supervisory Authority, the country's financial watchdog.
German stock market law requires publicly traded companies to alert investors as soon as they have unforeseen developments that could affect a decision to buy or sell the stock. Prosecutors said that Volkswagen only made that notification on Sept 22, and that there was evidence that the disclosure obligation should have been fulfilled earlier.
The company said the review showed no reason not to recommend that shareholders vote to approve management's work for 2015 at the annual meeting on Wednesday.
Volkswagen has said Winterkorn was sent a memo on May 23, 2014, about emissions irregularities uncovered by an environmental group, but the company was not sure he saw it, and said that top officials discussed the matter on July 27, 2015.
On September 18, the US Environmental Protection Agency issued a violation notice, leading Volkswagen to assess the risks as more serious and issue its investor advisory four days later.
The prosecutors' news release said that the second employee is not the current board of directors' chairman, Hans Dieter Poetsch. Poetsch was chief financial officer under Winterkorn but has since left that post.
The company has apologised and commissioned a law firm to investigate. It is negotiating a settlement with US authorities in federal court in San Francisco on how it would fix or buy back some 500,000 diesels sold in the United States. Some 11 million such cars were sold worldwide.
