A local sheriff called the suspect a “lunatic” and his office added the encounter did not affect the safety of Trump or the rally’s attendees.
Many questions remain unanswered.
While Riverside County Sheriff Chad Bianco said it was impossible to speculate about what was in the mind of the suspect, whom he called a “lunatic,” he said he “truly believed” that his officers had prevented a third assassination attempt.
He added that it might be impossible to prove that this was the man’s intent.
A federal law enforcement official told CBS News there was no indication of an assassination attempt connected to this incident.
Federal authorities say they are still investigating the incident, and it would be up to them to pursue any additional charges.
The incident – which occurred at 16:59 PDT (00:59 GMT) – an hour before Trump was scheduled to appear on stage – highlights, once again, the intense security operation around him, and the dangers facing the former president, with just over three weeks to go until the election.
It follows two high-profile alleged assassination attempts on Trump earlier this year.
Mr Miller was was charged with two misdemeanour weapons charges and was released on a $5,000 (£3,826) bail. No federal charges have been filed.
In a police news conference earlier on Sunday, Chad Bianco, a Riverside County sheriff, warned he might not be able to “give all of the information… because of what we’re doing”.
He dismissed a question about the suspect’s political affiliation – and instead said “he was a lunatic”.
The sheriff added that as the suspect approached an outside perimeter, near the location of the rally, he “gave all indications that he was allowed to be there”.
But as the suspect got to the inside perimeter, “many irregularities popped up”, Sheriff Bianco added, explaining that the vehicle had a fake licence plate and was in “disarray” inside.
Multiple passports with multiple names and multiple driving licences were found in the car, the sheriff said, adding that the licence plate was “home-made” and not registered.
