There have been vaguely mentioned hints that he did not act totally solo. Urged? Assisted in some way with "logistics"? Prior knowledge beyond fantasizing about a publicly announced appearance? The hints have been there pretty much from the start. One (very obvious) line of inquiry would be the whole shooting process, from when/how he learned to shoot and range-in a scoped rifle through his obtaining each part of his set-up (a Mauser bolt-action is not probably not rare, even in .30-06, but they probably are not cheap, the scope, the rounds he used ...), how he was able to get to his perch, how he got a long gun on campus, etc..
Unlike the LIEden FBI's "Missed him by that much" "investigation" of the Butler, PA assassination attempt, I think the FBI and locals have probably been/are diligently investigating these aspects.