Federal agents say Sandford told them he drove to Las Vegas from San Bernardino, California, with a plan to kill Trump.
He said he went to a Las Vegas gun range to learn to shoot the day before the incident at Treasure Island Hotel and Casino on June 18.
His father Paul Davey, 50, who traveled to the US to speak to his son in jail on Friday, said he was deeply shocked by Michael’s confession.
Mr Davey said: ‘I asked him what happened and he would only say that if Trump was elected, it would change the world and that somebody had to stand up for America.
‘I have never heard him talk like that before. I can’t understand why he was so motivated and politicized that he thought grabbing a gun from a policeman was a good idea.’
Mr Davey said, after spending an hour talking to his son that he believes Michael was brainwashed.