I think a thorough investigation can still take place AND she can answer basic questions about HOW this happened. It does not take witness interviews to explain why that building was not in the perimeter and why a sniper wasn't on that roof. Someone just raised a great point in this hearing that 60 days is too long with this being an election year and the country needs to know that this isn't going to happen again. Of course do the full and complete analysis, but there is no analysis needed to state facts about who decided the perimeter for that event, who decided how many SS were needed, why was SS not part of the morning briefing about the event, why was the perimeter not larger to include those buildings when they were close enough to the protected person and with a direct line of sight to them, and why was Trump allowed to take the stage.
It doesn't matter exactly who was doing what when on the ground and when the gun was put there and where was the ladder, and who's bike, etc etc.. they don't need to comb video evidence and interview everyone at the rally before stating these basic facts. Those facts are what lead to a man being able to have a gun that close to the protected target. When he obtained the gun, what he did that morning, who saw him when, etc doesn't really matter because NONE of that would have had any impact on Trump if the shooter wasn't allowed to be on that roof. If that rooftop was within the protected permitted then he would have no opportunity to get to that vantage point when a weapon of any kind.