For most of the Ukraine-Russia war, Western officials have been saying Russia’s been expanding a secret sabotage campaign aimed at European arms and defense manufacturing. This includes a string of suspicious fires, like the one that took out a metal factory for the defense company Diehl in Berlin last summer. Now, Western security officials are dropping some big claims about a Russian plot to take down cargo and passenger planes leaving Europe.

Security officials have stated that Russia tried to use incendiary devices shipped via DHL to start fires on planes flying to the U.S. and Canada. In July, the Wall Street Journal wrote an article about two electronic devices in packages that actually caught fire at separate DHL hubs—one in Leipzig, Germany, and another in Birmingham, England.

Poland has arrested four suspects on terrorism charges related to the fires at these DHL hubs, saying the suspects worked for a foreign intelligence agency. According to Poland’s National Prosecutor’s Office, the plan was to test if these incendiary packages could make it to North America undetected. The head of Poland’s foreign intelligence agency, Pawel Szota, strongly hinted that Russia’s military intelligence agency, the GRU, was behind it. He warned that Russia’s leaders might not understand the consequences of a mass casualty event if one of these packages actually went off.

The Kremlin, of course, denied everything. Their spokesperson, Dmitry Peskov, brushed off the accusations as “traditional unsubstantiated insinuations from the media.”

European investigators now agree the devices—electric massagers with a flammable substance inside—were likely a test to see if they could get incendiary devices onto planes to North America.

But there are still big questions. Could these devices have just malfunctioned by accident, especially since lithium-ion batteries can catch fire? And how strong is the evidence linking the suspects to Russian intelligence? While there have been accusations of Russian sabotage before, some cases haven’t held up well under scrutiny.

Plus, why would Russia target civilian airplanes now? What would they gain by exploding passenger planes?