It appears that the MD killer targeted that news office over some previous grievance. It wasn't a random attack against anonymous citizens as in the more common mass shooting.

A mass killer often picks a target-rich environment. If you're not the first person he selects, you may have the opportunity to deliver a single accurate shot before he knows of your presence. My guess is that the shooter would suffer from tunnel vision and only see the person in his sights at that instant. You could be standing next to that victim and the gunman may not see you.