Sandford allegedly told investigators he practiced shooting at a nearby gun range the day before the rally.
When officials asked Sandford why he tried to take the officer's gun, he allegedly said, "To shoot and kill Trump."
The 19-year-old was transferred into the custody of the U.S. Secret Service. He's charged with attempting to commit an act of violence on restricted grounds.
This video includes images from Getty Images and clips from KTNV.