LAS VEGAS (AP) — A man arrested at a Donald Trump rally in Las Vegas told authorities he tried to grab an officer's gun so he could kill the candidate, according to federal authorities.

A complaint filed Monday in U.S. District Court in Nevada charges Michael Steven Sandford with an act of violence on restricted grounds. He's expected to appear in court on Monday afternoon.

Authorities said Sandford went to a Trump rally on Saturday at the Treasure Island Casino and approached a Las Vegas police officer to say he wanted an autograph from Trump. The report says Sandford grabbed the handle of an officer's gun in an attempt to remove it, and was arrested.

It wasn't immediately clear if he had an attorney. Las Vegas police say Sandford is 19.

Sandford had a United Kingdom driver's license with him at the time, according to the complaint. He told a special agent that he was in the U.S. for about a year and a half and lived in Hoboken, New Jersey, then drove to the San Bernardino, California area in his car before coming to Las Vegas on June 16.

Sandford told officers he had been planning an assassination for about a year and was convinced he would die in the attempt. He said he also reserved a ticket for a Trump rally in Phoenix, scheduled for later in the day, as a backup.

He told authorities that he went to the Battlefield Vegas shooting range the day before the rally and fired 20 rounds from a 9mm Glock pistol to learn how to use it. Police detectives who visited the range spoke with an employee who confirmed that he provided Sandford shooting lessons, according to the complaint.

About 1,500 people attended the rally, which was held in the Mystere Theater inside the casino. Attendees had to pass through metal detectors manned by Secret Service, police and casino security officials.