"Winterkorn either knew of proceedings in the US or it was not reported to him," Evercore ISI analyst Arndt Ellinghorst said. "In the first instance, he must step down immediately. In the second, one needs to ask why such a far-reaching violation was not reported to the top and then things will get tough too." Analysts had been hoping Winterkorn's victory in his power struggle with Piech would allow him to focus on tackling the carmaker's long-running underperformance in North America as well as sharply slowing demand in its main profit centre, China. Scandal spreads The US Environmental Protection Agency (EPA) said on Monday it would widen its investigation to other automakers. Media reports say the US Department of Justice has also started a criminal probe into the allegations against Volkswagen, which cover VW and Audi-branded diesel models including the Audi A3, VW Jetta, Beetle, Golf and Passat.