Bollore's name was officially on the list of those being investigated for alleged market manipulation.
Bollore's name was officially on the list of those being investigated for alleged market manipulation, prosecutor Fabio De Pasquale told AFP.
Vivendi initially acquired a three-percent stake in Mediaset on December 12 and announced it aimed to increase it to around 20 percent, a move which Mediaset's owners, the family of former prime minister Silvio Berlusconi, rejected as "hostile".
The Berlusconi family, via its holding company Fininvest, asked justice officials to investigate possible market manipulation.
They accuse Vivendi of driving Mediaset's share price down by pulling out of a deal in July to buy the Italian broadcaster's pay-television unit Mediaset Premium.
Vivendi said it had second thoughts about the deal after Premium recorded losses in both the first and second quarters, offering instead to take a 20-percent stake in Premium and a 15-percent stake in Mediaset.
Vivendi has since built up a stake of nearly 30 percent in Mediaset with the aim of forcing the Berlusconi family into an alliance.
The French group is looking to strengthen its presence in southern Europe, and Spain and Italy in particular, via Mediaset and set up a platform for audiovisual content along the lines of Netflix.