Who is jamming aircrafts’ GPS in the Baltic?

.ON APRIL 29TH Finnair, Finland’s nationwide airline company, claimed that it would suspend flights to the asian Estonian urban area of Tartu for a month. Recently two airplanes were pushed to shift after their general practitioner signals were interrupted. General practitioner jamming has boosted dramatically in the Baltic area over the past pair of years.

Who is behind it and exactly how dangerous could it be actually?