Zen, I thought of that very thing this morning when I heard what time it happened..
Imo, he totally premeditated this mass murder, and probably had for awhile.
Most mass murderers usually does great planning before they kill their victims without warning right out of the blue. IMO, he used the surprise attack knowing they would have their guard down when he stuck Imo, he knew people would be in the parking lot on break. He knew he could pick off victims, one by one, even before he entered.
Once the FBI thoroughly investigates what's on his communication devices it will be quiet revealing, imo. He could have written out his to do list or left some kind of manifesto. He also may have been on some SM site.
Imoo