Back in 2015, Winterkorn was initially named by prosecutors into their investigation into wrongdoing at the company. The prosecutors quickly reversed course and said they were hasty in naming Winterkorn. Since they've now reversed on their reversal, it seems that German prosecutors feel that they have solid evidence that Winterkorn was complicit in the fraud. Of course, they'll have to prove that in court.
Winterkorn is just one of a total of 37 people that the Times reports the prosecutors are investigating.
VW recently has settled a number of aspects of its diesel cheating scandal, including a $4.3 billion settlement with the US Department of Justice over civil and criminal penalties related to cheating.