The essential details are these. Not long after Trump claimed that a surge in Latino voting in Nevada was evidence of voter fraud, a man named Austyn Crites (later self-identified as a registered Republican who opposes Donald Trump) was in the arena, relatively near the front of the audience. There was some commotion. Trump noticed the commotion, accused Crites of "being from the Hillary Clinton campaign."
From the stage he asked Crites, "How much are you being paid? Fifteen hundred dollars?" and then called for security to "take him out."
(The idea that the Clinton campaign sends people to Trump rallies to instigate violent disruptions is an urban legend growing out of the latest James O'Keefe tape dump. There is zero evidence to support this. It is a sort of mass psychology version of projection.)
At this point Crites was apparently in the process of pulling out a sign of some sort which someone nearby thought was a gun. That person yelled "gun!" This tripped off a melee in which Trump supporters beat Crites fairly severely. Secret Service agents, seeing the melee and possibly hearing the cry of "gun", rushed Trump off the stage and took Crites into custody.
It's easy to knock the people in the room for apparently freaking out over nothing more than a sign. It likely points to the febrile atmosphere within the Trump world, and especially among supporters at rallies, that such a tinderbox of fear and emotion could have blown up so quickly. Trump rallies frequently escalate to mob psychology events stirred up by fears of domestic enemies and external threats. Trump primes them for a blow up like this. (Here's a very good article on the subject.) Yet, it's also important to remember that America has a long history of political violence and specifically a long history of assassinations of presidents and presidential nominees. It made perfect sense for the Secret Service to escort Trump off the stage until they were confident there was no threat and that the area was secure. We should also bear that history in mind if we find ourselves chiding people for jumping to conclusions in the heat of the moment.
In any case, it was clear very quickly that there was no gun and that there was no threat. How do we know that? Because they allowed Trump to return to the stage very quickly. If there had been a gun or if they were not close to certain there hadn't been one, that would not have happened. The presence of a gun would have meant security had been breached and that likely would have been the end of the event. One gun can mean another gun. There are many examples of assassinations and assassination attempts were an initial commotion is used to distract from a subsequent attack etc. Letting Trump back on the stage only a few minutes later essential confirms the Secret Service knew very quickly that there had been no threat.
In other words, a jittery but perhaps understandable misunderstanding rapidly got out of control but was quickly determined to have been a misunderstanding. That's where it started to get ugly.
As I said, it was determined very quickly that nothing had happened. No attempt. No nothing. But this didn't stop the campaign from pushing out a storyline about an "assassination attempt" and a tale of Trump's bravery in immediately returning to the stage.
Next a CNN journalist went out from the press pen into the area where the incident had occurred to find out what happened. He was promptly verbally abused and physically assaulted, though seemingly to no great physical harm, mainly just shoved around.
— Richard W. (@IceManNYR) November 6, 2016
Things got darker still when Trump arrived a short time later in Colorado. In Denver, Trump was introduced by Father Andre Y-Sebastian Mahanna, a Maronite Catholic priest who said Trump had just survived "an attempt of murder against Mr Trump."
He then blamed the press for incitement the non-existent assassination attempt.
The Trump campaign allowed this to happen and made no effort to correct the record. This was followed by another warm up speaker who joked about Clinton being a 'bitch.'
By the end of the evening, Trump was fine. Crites was released little more than an hour after the incident - another clear sign that he had never been any threat to Trump. But core Trump supporters, immune from accounts of what had happened, been reported and verified, were off and running with a new fable about how Trump survived an attempted assassination. As I saw David Frum note in passing this morning, it is amazing the degree to which abusers are able to transmute their abuse into victimization, creating a grievance perpetual production machine. This is what the Trump campaign is.