Western security officials reportedly said they believe Russia is plotting to plant explosives aboard either commercial or passenger airplanes bound for the United States and Canada.
Two incendiary devices were shipped using the German logistics company DHL and ignited at DHL logistics hubs in Leipzig, Germany, and in Birmingham, England, in July, sparking a multinational investigation, according to the Wall Street Journal.
Security officials and sources familiar with the investigation told the Journal that intelligence agencies in Europe determined the explosions were caused by electric massagers implanted with a magnesium-based flammable substance.
Those officials say the electric massagers, sent to the United Kingdom from Lithuania, were intended as a "test run" of a wider Russian sabotage plot meant to evaluate how best to get explosives on board aircraft headed to North America.
Poland’s National Prosecutor’s Office announced that four suspects have been arrested in connection to the fires at the DHL hubs and are charged with "sabotage or terrorist operations on behalf of a foreign intelligence agency," the Journal reported.
"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, without revealing the names or nationalities of the suspects.
The leader of Poland’s foreign-intelligence agency, Pawel Szota, told the Journal that Russian spies were responsible and the actualization of any such attack would be seen as a "major escalation" of Russia’s sabotage campaign against the Western powers.
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"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," Szota told the Journal.
The Journal asserted that Szota’s comments and those of Western intelligence officials support the claim that the Russian military-intelligence agency, the GRU, was behind the plot.
"We have never heard any official accusations" of Russian involvement," Kremlin spokesman Dmitry Peskov told the Journal when asked for comment. "These are traditional unsubstantiated insinuations from the media."
The U.K.’s counter-terrorism police is investigating the Birmingham fire and is working with other agencies in Europe. German police reportedly tested replicas of the incendiary devices and said that the firefighting systems on most planes would struggle to extinguish the magnesium if it ignited on board.
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Sources familiar with the German investigation said pilots would need to make an emergency landing in that scenario or the plane could go down over water if an immediate landing isn't possible.