There were perhaps five other people in the car with the Secret Service agent carrying the AR-15. If that weapon fired and they said nothing, they are also part of the coverup.
I'm not going to dispute that. If I'm not mistaken, there is an even greater cover up going on in Washington right now that is trying to be unraveled.
As a side note, there were numerous attempts by Menninger and Donahue to interview the agent but were refused.........Is that significant? I don't know, I'm sure the agent had his reasons not to talk to them.