I saw a report on CNN (youtube) that the missile that landed in Poland was the result of an attempted shoot-down of a Russian missile that then went off course*. I don't believe it, but my thoughts count for little. If it turns out that this *was* a missile launched by the Russians, (again, my opinion) the best response might be the authorization of the transfer of a batch of ATACMS to Ukraine. They would have the range to reach the launch sites of a lot of the (non-air launched) missiles, and some air bases fielding bombers.
*The airspace over Ukraine has to be one of the most heavily monitored regions of the world right now, especially by the US and allies. There are many air, sattelite, and ground based assets involved. We either know (or will shortly know) exactly what happened, and will likely have proof. I doubt the proof will ever be made public as this will reveal our capabilities.