The assassin should never have had the opportunity to get a shot off. Therefore, the Secret Service was massively incompetent even negligent (99.9% likely) or there was a conspiracy (0.1% chance).
Back in the day shortly after Gerald Ford became president I stood along a road in East Grand Rapids, MI to get a glimpse of the new president along with hundreds of others. A couple Secret Service agents approached a man standing nearby and searched and questioned him. We surmised he looked suspicious because he had a long mustache, sideburns and was wearing a leather jacket. Hard to imagine how someone pacing around with a rangefinder and backpack doesn't get stopped.