The metal box, which was snatched from offices of Bulgaria's Novinar daily and the Vsekiden.com news website, contained documents dealing with investigations of journalists from the media.
This was announced by the two media manager Miroslav Borshosh.
"The investigations concern a circle of companies of the so-called "Bulgarian oligarchs" and their relations with political parties," Borshosh commented.
In the two assaults, which took place Wednesday night, the attackers forced the offices' doors open and tried to carry out two large safety-vaults.