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The sniper was outside the Secret Service bubble—outside the venue on a roof.
The problem is that witnesses outside the venue saw the perp and pointed him out to local police and SS agents who failed to act—this was going on for a couple of minutes.
At a minimum they should have notified SS inside the venue and escorted President Trump off the stage until the (potential) perp had been captured and removed from the roof.
Do we know that he was inside the bubble?