Several dozen schools across Bulgaria were ordered shut on Monday and Tuesday after receiving emailed bomb threats with suspected links to Russia, police said.
"The main lead of investigators is that these are hybrid attacks somehow linked to Russia," Interior Minister Ivan Demerdzhiev said.
Bulgarian authorities have requested assistance from its partners' intelligence services -- in particular from the United States -- in an effort to identify the perpetrators of the threats, he added.
The emailed bomb threats originated from Google accounts, authorities said.
One such message seen by AFP read: "There are TNT charges placed in all schools. Soon all your schools will explode. I hope you die."
The prosecutor's office said it was probing a crime against the republic, or terrorism, and was working to establish the source of the attacks.
The alerts prompted the evacuation of dozens of schools in the capital Sofia, the Black Sea cities of Burgas and Varna, Sliven, in eastern Bulgaria, and Pleven, in the north, causing panic among the public.
So far no explosives have been found in the schools.
In total, 102 bomb threats have been recorded since Monday.
The attacks come just days ahead of general elections in Bulgaria on Sunday, with most polling stations being set up in schools.
Yet, voting would go ahead as planned unless some real threat were to arise, Demerdzhiev said.
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© Agence France-Presse
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