SOFIA, Bulgaria (AP) — Dozens of schools across Bulgaria were closed and their students evacuated for a second consecutive day following bomb threats that were made by email and phone.

Most of the threats received Tuesday were directed at schools in the capital, Sofia, and in the Black Sea ports of Varna and Burgas. Students were evacuated for 24 hours, and police stepped in to search for explosives.

No bombs were found so far, authorities said.

Interior Minister Ivan Demerdzhiev said partner agencies in Europe and the United States were helping to establish the source of the threats. Similarly worded warnings were sent to schools in other European countries, he said.

The series of bomb threats come amid preparations for Bulgaria’s general election on Sunday. Most polling stations are located at schools.

Yavor Kolev, a cyber crimes expert, thinks the bomb threats were most likely linked to a hacker group involved in political extremism.

“These are hacker groups engaged in political extremism, instilling panic and fear in society. From the analysis of the materials published in media, I can conclude that the attack is foreign and most likely coming from the Russian Federation,” Kolev told Bulgarian National Radio.

Bulgarian authorities plan to review the protocols for analyzing potential dangers before proceeding with the evacuation of public institutions.