My sympathies go out to the cop. Every night he probably wakes in the early hours in a cold sweat - what might have happened. A life sentence for him if not for the perp.
Some oddities about that... he crossed the country but left no record of where he stayed; he was an unknown; etc. — ie, a perfect “loner” for an assassination story.
Hypothetical:
He couldn’t bring a weapon into a rally — that would be detected; so assume the “plan” was for him to “grab” a gun once inside, from a co-cooperating cop.
But he was only there to create a distraction (ie, attempting to shoot). The real intended shooter (another cop, or guy in a cop’s uniform) would do the actual shooting (”crossfire”).
That’s a typical mafia-style tactic.
Of course he went to the wrong cop, and the job was botched.
Maybe none of that happened, and it’s only tin foil hat stuff. But given our history since 1963, nothing can be ruled out.