Bloody arms shown on TV (video transmitter) are used for proof about a dramatic event. Like the manipulated news on mass media.
When Ethan holds the murder weapon in the beginning, he already has all the clues he – and we, as the viewers – will need to uncover the truth. It’s another sly game on behalf of De Palma. When Ethan holds Krieger’s hand, itself holding the knife, he makes it abundantly clear that he’s on a new path and will not cross moral boundaries. Also, we might have a clue for the puzzle.
In the fabulous shot where the camera zooms on the speeding train and shows the window, we can see only the hands of the manipulator, but we’re still not sure whom they belong to.