Vodafone boss Chris Gent is being investigated by Germany's public prosecutor over alleged irregularities in the company's takeover of Mannesmann.
Germany's public prosecutor says it is also investigating former Mannesman chief executive Klaus Esser and a Deutsche Bank executive.
Mannesmann was bought by Vodafone last year after a fierce battle for control in what became the world's biggest ever hostile takeover.
Deutsche Bank management board member Josef Ackermann is named as being one of those investigated, as is IG Metall trade union head Klaus Zwickel.
Deutsche Bank has refused to comment.
Vodafone later sold Mannesmann's mobile phone division - Orange - to France Telecom.
Investigators are examining the pay-offs to the former Mannesmann executives to determine whether they were too high or not commensurate with the executives' duties or the financial state of the company, as required by German share law.
According to German magazine Der Spiegel, there are suspicions that the payments were in effect bribes, aimed at compensating Mannesmann executives for dropping their resistance to Vodafone's hostile bid for the firm.