According to a report by the , intelligence officials believe Russia is plotting to plant explosives on commercial or passenger airplanes headed to the United States and Canada. 

In July, two incendiary devices were shipped using DHL, a German logistics company, and ignited at DHL logistics hubs in Leipzig, Germany, and Birmingham, England. The incidents sparked a multinational investigation, according to the .

Security officials and sources familiar with the investigation told the Journal that European intelligence agencies determined that the explosions were caused by electric massagers implanted with a flammable substance made of magnesium.

These officials say that the electric massagers, shipped from Lithuania to the United Kingdom, were meant as a “test run” of a wider Russian sabotage plot. The plot aimed to assess how best to get explosives onto aircraft flying to North America. 

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.   

Pawel Szota, the head of Poland’s foreign-intelligence agency, told the Journal that Russian spies were responsible. He added that if an attack were to occur, it would represent a “major escalation” of Russia’s sabotage campaign against Western powers. 

“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, as well as those of Western intelligence officials, support the claim that the GRU, Russia’s military-intelligence agency, was behind the plot. 

“We have never heard any official accusations” of Russian involvement,” Dmitry Peskov, Kremlin spokesman, told the Journal when asked for comment. “These are traditional unsubstantiated insinuations from the media.”

The U.K.’s counter-terrorism police are investigating the Birmingham fire and are 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. 

Sources familiar with 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.