Who is waging hybrid warfare in Eastern Europe?

On Sunday, Serbian President Aleksander Vučić announced that two backpacks containing explosives were found close to the Hungarian border near the Balkan Stream gas pipeline, which supplies Hungary and Serbia with crucial Russian gas. Hungary’s prime minister implied that Ukraine was the culprit, but Ukraine denied it and called it a Russian attempt to interfere with Hungary’s election on Sunday. If this is true, it will further stir Europe’s fear of Russia, which has allegedly conducted other recent sabotage attempts in Eastern Europe.