Russia was responsible for sending two incendiary devices to DHL logistics hubs in Germany and the United Kingdom in July as part of a wider sabotage campaign to possibly start fires aboard aircraft bound for North America, a Western security official told NBC News.

The Wall Street Journal first reported on the suspected plot.

Polish authorities have arrested four people and are searching for two more in connection with the case, according to government officials and Polish media.

DHL said in an email that it is “aware of two recent incidents involving shipments in our network” and that it is cooperating with investigators.

The U.S. Transportation Security Administration said it has bolstered air cargo screening over the past several months. 

A U.S. official said federal authorities remain vigilant against threats to aviation and air cargo systems and added, “At this time, there is no current active threat targeting U.S.-bound flights.”

The White House National Security Council and the CIA declined to comment. 

German and British authorities have said they were investigating suspected Russian sabotage in two incidents in July. A package en route to an aircraft exploded at a DHL logistics center in Leipzig before a scheduled flight. Also in July, a package burst into flames at a DHL center in Birmingham, England. 

The packages included electric massagers that contained a magnesium-based flammable substance, the Western official said. 

Poland’s national prosecutor’s office said the saboteurs’ goal was “to test the transfer channel for such parcels, which were ultimately to be sent to the United States of America and Canada.”

Last month, Thomas Haldenwang, the head of Germany’s domestic intelligence service, told German lawmakers that it was sheer luck that one of the booby-trapped packages “caught fire on the ground and not during the flight of the plane.” He said the incident was “a particularly great danger. “

U.K. counterterrorism police investigating the incident declined to comment on the DHL cases.

In recent months, U.S., British, German and other European officials have accused Russia of organizing an increasingly aggressive sabotage campaign aimed at undermining and disrupting Western assistance to Ukraine. 

The attempted sabotage has included a plot to bomb or set fire to military bases in Germany, attempts to hack and disrupt Europe’s railway signal network and the jamming of GPS systems for civil aviation, according to European and British authorities. The physical sabotage efforts have coincided with a concerted disinformation campaign by Moscow to weaken political support for Kyiv in the West, officials say.

The director of the U.K.’s MI5 domestic intelligence agency, Ken McCallum, said is a speech on Oct. 8 that Russia has turned to private intelligence operatives and criminals to do “their dirty work.”

Russia’s GRU military intelligence service “in particular 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,” McCallum said.

Russia has repeatedly rejected allegations from Western countries that it is orchestrating a sabotage campaign in Europe.