The bus driver was about to start his morning run the article says. The run according to the schedule I just posted started at 5:18am. This wasn't called in until about 5:45am as I understood it. This shooter had plenty of time to leave the city before someone found the bus driver.
So far, no mention of witnesses at the time of the shooting.
ABC radio news reports that the roadblocks did not go up until some 20 minutes after the shooting. That seems to help prove your theory.