Authorities say a gunman who fatally shot five people at a Washington state mall initially entered the shopping center without a weapon, but 10 minutes later, went inside Macy’s with a rifle and opened fire.
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http://nypost.com/2016/09/24/authorities-find-hunting-type-rifle-from-mall-shooting-scene/
I'm inclined to believe this report over the previous one. Sounds like they obtained the info from LE.
So either he came into the mall earlier with the gun and hid it somewhere or he worked at the mall and brought it in and hid it at some point or someone else brought it in for him.
I would exclude the second one because if he worked there someone would have identified him already.
So where did he go in those 10 minutes?