The public prosecutors office in the German city of Düsseldorf is investigating the role played by the renowned Swiss banker, Josef Ackermann, in Vodafone's takeover of the Mannesman Group.
Ackermann is now head of investment banking for Deutsche Bank and was a member of Mannesman's supervisory board at the time of the takeover.
German prosecutors are investigating whether there is enough evidence to proceed with charges after accusations that bonuses paid at the time were intended to influence the course of the takeover battle.
The Düsseldorf prosecutor said on Monday that the investigation into nearly DM150 million (SFr116 million) of redundancy and pension payments to former Mannesman directors had been extended to Ackermann and to Klaus Zwickel, the head of the powerful IG Metall trade union.
Vodafone insists that no contact was made with Mannesmann board members before their acceptance of the bid. The British company says it had little choice but to ratify bonuses once it became clear the takeover would go through.
The probe has provoked an uproar in Germany because of the sums involved. Severance payments and managers' salaries are lower than in many other countries.
Such investigations, however, rarely lead to formal charges.
swissinfo with agencies