Actually, Occam's Razor would lead me to believe that there were two shooters working in concert.
The police officer speaking at the press conference just now hinted that the incidents may have involved two different shooters.
Experience would point to a planned, multi-party, event, IMO.
He causes a panic at 7:15AM...and calmly walks out the door just like the hitman did in last nite's Sopranos episode. He walks over the Norris Hall and hides someplace where he had already placed his cache of weapons and ammo.
The university locks down the building with him already inside...or he chained the doors himself.
Once the hallways are almost empty with 900 students in the building, he starts his mayhem.
This is Sudden Jihadist Syndrome at a minimum...or a planned attack.
Let's hope another one doesn't happen this week, folks.