At the same time in Detroit, GM said it was co-operating with US Justice Department officials in their investigation of possible document theft by former GM employees.
According to sources close to the German investigation, the Darmstadt prosecutor's office has come into possession of new material that was among confidential information missing since the departure of Mr Lopez in March. The material had apparently been in the hands of a former employee of Opel, the GM subsidiary, and had been brought to the state prosecutor by someone else.
Statements by senior GM executives interviewed over the past two days concerning another set of secret documents found earlier in a Wiesbaden flat rented by two of Mr Lopez's associates have also 'substantially strengthened our suspicions', Georg Nauth, spokesman for the Darmstadt prosecutor's office, said. He said the prosecutor wanted to speak to Mr Lopez and the two connected with the Wiesbaden flat, Jorge Alvarez and Rosario Piazza, but apparently all were away on holiday.
Volkswagen's shares for the first time yesterday reflected growing nervousness about the turn of events, falling DM4.5 to DM361 in London. Frankfurt traders spoke of rumours spreading among American investors that Mr Lopez's days were numbered.
'It looks increasingly as if Lopez actually did take the documents when he moved from GM, and the question we must ask ourselves is, did Ferdinand Piech know about it?' one trader said, referring to the chairman of Volkswagen who poached Mr Lopez from Detroit.
The nervousness was provoked by a civil court hearing in Hamburg on Thursday when several senior GM executives contradicted under oath statements made by Mr Lopez about the collection of sensitive documents before his departure for VW.
The Hamburg hearing did not directly concern GM but was prompted by Der Spiegel magazine seeking to overturn a temporary injunction by Volkswagen banning it from pursuing allegations of industrial espionage by Mr Lopez. However, GM seized the opportunity to strengthen its case against VW, sending 22 executives, four corporate lawyers and bagfuls of sworn affidavits.
Many of these executives have now been interviewed by the Darmstadt prosecutor, who is dealing with the more serious criminal investigation against Mr Lopez. Mr Nauth said the executives' statements confirmed that the GM papers found in the Wiesbaden flat 'are of great importance to the firm, and should not have been there'.Reuse content