Two incendiary devices transported by DHL were part of a Russian plot to start fires on cargo or passenger planes headed to the US and Canada, the Wall Street Journal reports.

The current plot shows an escalation of previous suspected attacks in July when devices set fires at DHL logistics hubs in England and Germany.

Since the incidents in July, security officials have been searching to find the culprit and have now determined that Russia was behind the planting of incendiary devices, according to the Wall Street Journal.

The devices were electric massagers filled with a magnesium-based flammable substance, which would be difficult to extinguish if ignited on an airplane.

Poland’s National Prosecutor’s Office has arrested four people in connection with the fires and has charged them with terrorist actions on behalf of a foreign intelligence agency.

“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,” the prosecutor’s office said.

Pawel Szota,  the head of Poland’s Foreign Intelligence Agency, said Russian agents were responsible for the plot to plant the devices, and if an attack had been put into action, it would represent a significant escalation from the incidents in the summer.

Szota said, “I’m not sure the political leaders of Russia are aware of the consequences if one of these packages exploded, causing a mass casualty event.”

When the Wall Street Journal asked Kremlin spokesman Dmitry Peskov to comment on the incident, he replied, “We have never heard any official accusations” of Russian involvement, adding: “These are traditional unsubstantiated insinuations from the media.”

European authorities have noted that Russia is behind an increasing number of cases of sabotage, including arson in the UK and Finland, tampering with pipelines and data cables in the Baltic, and interfering with water supplies in Sweden and Finland.