Porsche's offices have been raided by federal prosecutors probing the alleged market manipulation of Volkswagen shares.
"On Thursday morning, officers from Stuttgart prosecutors entered the company's offices with search warrants.
"The prosecutors suspect a breach of public disclosure requirements and market manipulation," a Porsche statement read.
Porsche had built up a stake of 51 per cent in VW in an attempt to launch a takeover of Europe's biggest carmaker. The two have subsequently agreed to a merger which will take place in 2011.
Porsche made huge profits on its VW shares, leading some commentators to describe the firm as a hedge fund with a carmaker tacked on the side.
The carmaker denied the accusations and said that it would co-operate fully with the prosecutors.
Reports by the BBC suggest that the investigation will focus on the roles of former chief executive Wendelin Wiedeking and finance chief Holger Haerter, both of whom resigned last month.
In order to shore up its finances, Porsche announced last week that the state of Qatar would take a major stake in the company.