us where that shot came from. >> some critics say by the very fact that in the picture you can clearly see the explosion of the bullet on the front side of the president, that that certainly indicates the bullet came from the front. >> well, i don't believe any physicist has ever said that. quite contrary, it does indicate that the bullet was coming from behind. it's a minor explosion where pieces of material go generally in the direction of the bullet. >> if you look at the individual frames of the zapruder film, at frame 312 the president's head's okay. at frame 313, 1/18th of a second later, the president is struck in the head. and what direction is the president's head pushed? not backwards but slightly forward. >> is there any doubt that the wound at the back of the president's head was the entry wound? >> there is absolutely no doubt, sir.