Who is actually jamming aircrafts’ GPS in the Baltic?

.ON APRIL 29TH Finnair, Finland’s national airline, mentioned that it would certainly suspend air travels to the asian Estonian city of Tartu for a month. Recently 2 aircrafts were forced to reverse after their family doctor signs were interrupted. GPS jamming has boosted sharply in the Baltic location over recent pair of years.

Who is behind it and how unsafe could it be actually?