A series of parcel fires targeting courier companies in Poland, Germany and the U.K. were test runs for sabotage of cargo flights to the U.S. and Canada, orchestrated by the GRU, Russia’s military intelligence agency, Polish prosecutors say.

BBC News reports that Katarzyna Calow-Jaszewska from Poland’s National Prosecutor's Office said that four people had been arrested and authorities across Europe were investigating the “activities of foreign intelligence by committing acts of sabotage consisting in damaging industrial facilities and critical infrastructure in the form of airports, aircraft and vehicles, and initiating fires using courier parcels with spontaneous combustion.”

Russia denies being behind acts of sabotage. But it is suspected to have been behind other attacks on warehouses and railway networks in EU member states this year, including in Sweden and in the Czech Republic.

“The group's activities consisted of sabotage and diversion related to sending parcels containing camouflaged explosives and dangerous materials via courier companies to European Union countries and Great Britain, which spontaneously ignited or detonated during land and air transport,” said Calow-Jaszewska said in an October 25 statement. “The group's goal was also to test the transfer channel for such parcels, which were ultimately to be sent to the United States of America and Canada.”

On three days in July, fires broke out in a container due to be loaded on to a DHL cargo plane in the German city of Leipzig, at a transport company near Warsaw, and at Minworth near Birmingham, U.K., all involving a package described as an incendiary device.The incident at Jablonow near Warsaw took two hours to extinguish, according to Polish reports.

Ken McCallum, head of the U.K.'s domestic intelligence agency MI5, said last month that the GRU — the Main Directorate of the General Staff of the Armed Forces of the Russian Federation, formerly the Main Intelligence Directorate — is “on a sustained mission to generate mayhem on British and European streets. “We’ve seen arson, sabotage and more. Dangerous actions conducted with increasing recklessness" in retaliation for the U.K. helping Ukraine in Russia's war.

In Germany, the head of the domestic intelligence agency Bundesamt für Verfassungsschutz (BfV) has said it was only by a stroke of fortune that the Leipzig device had not ignited in mid-air.