Shares in carmaker Porsche closed down 11% after it revealed that its merger with Volkswagen could be delayed.
Due to the widening of parts of an investigation into two former executives, Porsche said the likelihood of the deal being completed this year had dropped from 70% to 50%.
Prosecutors in the Stuttgart, Germany, have been investigating allegations of share price manipulation.
They said they would also now be looking into possible credit fraud.
Volkswagen shares also closed down, by 4%.
The outcome of the investigations is important for the deal as it will affect the amount of damage claims Porsche could face, and therefore the value of the firm.
Under the terms of its merger with VW, the deal was meant to have gained shareholder approval in 2011.
But prosecutors this week informed Porsche that their investigations would end at the beginning of 2012 at the earliest, the luxury carmaker said in a statement.
"In the view of Porsche... the overall probability of the merger decreases in case of substantial delays in the merger process," it said.
However, it added that it was of the opinion that the merger could still go ahead, even after the end of 2011.
Former chief executive Wendelin Wiedeking and former chief financial officer Holger Haerter face allegations of market manipulation.
Part of the original investigation has been dropped. But another part of it, which involves actions which the prosecutors allege "could have threatened the existence of Porsche", has been intensified.
Stuttgart's public prosecutor also said it was now also looking into suspected credit fraud, but did not say who was under investigation.