Russia denies it vehemently and initial pictures of the "device" that landed in Poland appear to show a S-300 anti-air defense missile.

The big unanswered question is whether this was an error, i.e., Ukraine fired at incoming Russian missiles and the S-300 missed its target and fell inside Polish territory, or was it an attempt to manufacture a false flag, i.e., blame Russia for firing the missile and create a pretext to invoke Article 5 of the NATO treaty and bring the rest of NATO into the war to stop Russia. The jury is still out.

The picture below shows first, one of the fragments of the S-300 that landed in Poland with a bang, and second, a fully intact S-300 rocket.