Two things:
One, I'm suprised the truck has not been located.
Two, there's lots of room in there to put the shooter up high, in the dark, and put lots of sound-absorbing material around him.
Three, there's almost got to be an accomplice driving, and possibly another accomplice operating the door on the van.
That's the way I see it too. A report yesterday that the last of yesterday's shootings took place while police were swarming around that very area indicated that the killer was a local psycho more interested in disrupting a given area than in his own safety. It is looking more and more, though, that two perps are involved, a shooter and a driver. That indicates terrorism.