They tried to get Proud Boys there so they can blame it on them.

When PB wised up and decided to not attend, they had to call an audible and craft another story.

They are going to target a soldier who lied about his support for Trump in this process. Kidnap him right before shot is taken and get his prints on the weapon. They will plant maps/plans on him, drug him, and leave him alone in a bunker with a gun. FBI will "track him down" and raid the bunker killing him before he even has a chance to speak.