Not US sources. It seem to comes from the Radio ZET post this points to
"Polish media reported that two people died Tuesday afternoon after a projectile struck an area where grain was drying in Przewodów, a Polish village near the border with Ukraine."
ZET seems a legitimate source, but it's still a single journalist and some twitter posts.
More coverage at
https://wiadomosci.radiozet.pl/Polska/przewodow-rakiety-spad...