Three men were detained by the Estonian Internal Security Service (ISS) on suspicion of forming an anti-Estonia association. These are early suspicions, which may change or be clarified as the case progresses.
Two Estonians and a Russian with a residence permit in Estonia are among the detainees. Aivo Peterson is one of them.
As the initial procedural steps are still ongoing for all three, the prosecutor's office said, it is not possible to give more details about the investigation at this stage.
The criminal proceedings are led by the State Prosecutor's Office and carried out by the the Estonian Internal Security Service.
Aivo Peterson, one of the most prominent members of the KOOS movement, ran as a candidate for the Estonian United Left Party (EÜVP) in the county of Ida-Viru and received 3,968 votes.
Peterson has previously made pro-Russia statements and visited Russia-occupied territories of Ukraine, from where he returned last this week.
Editor: Huko Aaspõllu, Kristina Kersa