Unidentified robbers have broken into the offices of Bulgaria's Novinar daily and the Vsekiden.com news website.
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.
Only one, smaller metal box, was stolen successfully.
"The metal box contained documents dealing with investigations of journalists from the media," announced the 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.
Novinar newspaper caused a lot of controversy in Bulgaria a couple of weeks when it ran 12 cartoons mocking Libya's leader Muammar Qaddafi. The cartoons aimed at countering Bulgaria's "quiet diplomacy" in the case of the five nurses jailed in Libya on accusations of infecting hundreds of children with the HIV virus.