Pavol Gašpar, head of Slovakia’s civilian intelligence agency (SIS), said that authorities had detained two people with potential links to espionage during a police operation on Tuesday in the town of Michalovce in eastern Slovakia.

Identifying the suspects as a Slovak with Hungarian citizenship and a Ukrainian, Gašpar said: “We have observed long-term, coordinated activities by a group of individuals potentially engaged in espionage.”

The detentions coincide with an investigation into a string of bomb threats targeting schools, universities and courts since spring.

Slovak media outlet Noviny reported that although the authorities have identified a suspect, an extraordinary session of the State Security Council was convened on Thursday to address the escalating concerns.

Interior Minister Matúš Šutaj Eštok said that the heightened security measures were to protect critical infrastructure.