Russia recently admitted to doing this. Wasn't that the reason the Polish delegation was flying to Moscow when it crashed. Wasn't there some connection to their visit having to do with the mass killings and an apology? Anyway as I understood it there was really no question as to who committed this heinous crime. I believe when the Nazi's initially discovered the bodies they brought in the Red Cross to verify the findings. (Ironic that) And when the Russians retook the area they had their own press day to point the finger at the Nazi's. But IIRC an astute daughter of the US Ambassador noticed something strange about the clothes on the bodies. They were the exact opposite of the type you would wear during the time of year that the Russians said they were killed.