No one was hurt in the explosion at Galeria, a weekly publication, Sofia News Agency reported.
In January, in an incident dubbed "Tapegate," Galeria released taped conversations of government officials speaking disparagingly of the Borisov administration. The tapes are believed to have been leaked by one of two ministries and their authenticity has been challenged, the news agency said.
The explosion prompted Galeria Editor Kristina Patrashkova to say the incident was meant to intimidate the newspaper.
"I am at least glad that the bomb was placed professionally so that no people were hurt," Borisov said during a joint news conference with European Union Commissioner for Health and Consumer Policy John Dalli. "At the same time, I am not the least worried by this incident occurring on a day when four EU commissioners are coming to Bulgaria because it is clear to everyone in Europe that my government inherited a country rife with organized crime and corruption, and that we have done a lot in the past months to tackle them."
The prime minister said the government could provide security for the Galeria offices only if the publication requests it; otherwise its staff could claim they are being tailed.