The leading theory about the bomb threats emailed to schools is that they are hybrid attacks linked to Russia, Interior Minister Ivan Demerdzhiev said Tuesday.
He stressed that the threats are also happening in other European countries with the same sources and approaches of attack.
The emails and phones from which the threats come are registered in the US and the UK, Demerdzhiev added.
He added that he would convene an inter-agency working group later in the day to review the protocol for action on receiving a signal of an explosive device so that potential dangers could be analysed before proceeding with evacuation of a school, airport or other infrastructure sites.
Ivan Demerdzhiev added that within days there will be a change in the protocol to make it easier to judge when there is a real threat.
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