Dozens of schools and a Sofia business university were closed following bomb threats on Tuesday just five days before Bulgaria holds parliamentary elections.
Prosecutors in the capital said they had launched an investigation into "crimes against the republic."
A police search failed to reveal any explosive devices at the schools, which had received the threats by email or phone since Monday.
Acting Interior Minister Ivan Demerdzhiev said the authorities were working on the assumption that the threats were part of "hybrid attacks" that could be linked to Russia in some way.
Bulgaria had appealed to investigating authorities in Europe and the United States for help in tracing the origin of the emails, Demerdzhiev said.
Many of the polling stations in Sunday's elections will be housed in schools across the country.
Demerdzhiev said the elections would only be interrupted in the event of genuine danger. He stressed that no real threat had thus far emerged from the information received.
