Who is congesting aircrafts’ GPS in the Baltic?

.ON APRIL 29TH Finnair, Finland’s national airline, said that it would put on hold flights to the eastern Estonian area of Tartu for a month. Recently 2 planes were actually required to turn around after their GPS signals were actually disrupted. GPS jamming has boosted dramatically in the Baltic region over recent 2 years.

Who is behind it as well as exactly how risky could it be?