Fourth day of bomb threats to schools in Sofia and throughout Bulgaria. More than 45 new threats, again sent by email, have closed schools. The police investigation aimed at determining the whereabouts of the sender continues. Site directors are deciding whether classes should continue, Bulgarian media reported.

Several schools in Burgas started the day with emails about explosive devices placed in the buildings, due to which the students were evacuated. Educational institutions in Plovdiv, Stara Zagora, Dobrich, Yambol and Sofia also received threatening messages.

"We don't know how to react to such signals," national security expert Milen Ivanov told Bulgarian media. "Even if it is not specifically hybrid warfare, creating psychosis is also not beneficial to society and somewhat helps to achieve the goals. We have a problem - we have a strategic framework Act for anti-terrorism, but, unfortunately, we don't have tactical actions, we don't know what to do," he explained.

Bulgaria's national counter-terrorism plan is expected to be updated by May 31.