CBS news reports that police sources have told them that neither man in custody is the sniper.
My opinion is: the shooter left the note. He/she then hired the white van drivers, via the use of something called "cut outs". The drivers of the van didn't know who hired them.. and the person who hired the van to drive there didn't know who told them to hire the van drivers.
The shooter(s) is/are milking this "white van" rabbit trail to the limit.