I am usually VERY skeptical of conspiracy theories.
That is because the number of people that would have to keep silent about their role is usually very large.
In this case, the number people who were "in on it" may be quite small. Maybe just 2 or 3 people. That's very plausible.

"Conspiracy" implies that someone on the local police or USSS side knew that a shooter would be there and made sure the path was clear for him. That's plausible. Not the most likely scenario but it is plausible.

The most likely scenario is that no one expected the shooter to be there, but some small number of people decided to "forget" their training about how to deal with a threat once they spotted the shooter. As I said, they are trained to evacuate and cover their protectee as soon as an immediate threat is identified. And I'm pretty sure almost any of them are authorized to call for evacuation to begin if they see a threat. That's what needs to be investigated first. Who should have made the call, why wasn't the call made.

---