The bomb at the Sofia Interpetrolium and Partners office building was most probably planted by an insider, Bulgaria's Interior Chief Secretary Boyko Borisov told private bTV channel.

On Monday a bomb explosion took place at the office building that houses Interpetrolium and Partners, Bul Ins insurance company as well as more than 10 other firms. Four people were killed and at least seven were injured in the blast.

Stoil Slavov, one of the owners of Interpetrolium and Partners, and his bodyguards Petar Petrov, Iliyan Kazakov, 37, and Petar Harizanov were identified as the four casualties.

The assault was carried out very precisely, General Borissov underlined.

When asked to comment on the rumors that Slavov was one of the men that gathered in Thessaloniki to plot Borissov's removal from the police structures, Bulgaria's Interior Chief Secretary declined to comment.

He once again urged for law reforms that will put all criminals behind bars and would give them no chance of getting out. If such reforms are not executed, murders likes Monday's one will soon happen again, Borissov explained.

Right after the Amsterdam killing of Bulgarian underworld boss Konstantin Dimitrov-Samokovetsa General Borissov revealed that Samokovetsa and another two notorious figures in the Bulgarian underworld planned to "remove" him. Borissov explained that the discussion for the "removal" took place at a meeting several months ago in Thessaloniki.