russian investigators are look at human error as the possible cause of a plane crash that has poland in mourning. the country's president was killed, along with dozens of top government and military leaders. cnn's fred plankan is in warsaw with the latest. >> reporter: as investigators piece together just what happened to poland's presidential plane, pols are trying to come to terms with how a single disaster could wipe out such a large number of the country's elite. >> translator: we can see that certain procedures were not adhered to. of political and military leaders should not all have been on the same plane in such numbers. of. >> translator: it is unimaginable for me, because so many generals and politicians died, and i can't imagine how so many of them could have gone on the same plane. >> reporter: the toll was staggering. the president kaczynski and his