Polish President Andrzej Duda said on Wednesday that the explosion that killed two people inside his country near Ukraine’s border was most likely an “unfortunate accident” and not an “intentional attack.”

He added that the projectile that caused the blast was “most likely” Russian-made, but that officials had “no proof at the moment that it was a missile fired by the Russian side.”

NATO Secretary General Jens Stoltenberg echoed the Polish President, saying the explosion was most likely caused by a defense missile fired by Ukraine while defending itself against the Russian strikes.

He stressed that “this is not Ukraine’s fault.”

In a statement on Tuesday, Russia’s defense ministry said it did not strike any targets in or near Poland.

If the strike were intentional, it would bring Article 5 of the NATO charter into consideration. It dictates that an attack on one member nation is an attack on all. It has only been invoked once, after the attack on the U.S. on September 11, 2001.

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