 To assassinate him, that he is resolved and he is strong. Investigators are still looking for the shooter's motive. National security contributor Samantha Vinograd. Blown actors are notoriously hard to detect and hard to prevent from engaging in violent activity. Secret service agents shot the 20-year-old gunman to death. CBS's Sabrina Franzen. Investigators say an AR-style rifle found near his body was purchased legally and registered to his father.