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Robin Westman -- the school shooter who killed two kids at a Minneapolis Catholic school last week -- went to a gun store days before the shooting ... and handled multiple weapons, security video from the shop shows.

New footage, obtained by KSTP in Minnesota, shows Westman walking into the store around 12:30 PM on Saturday, August 23 ... four days before the shooting at Annunciation Catholic school.

Westman walks around the store ... interacting with staff members and looking down the sight of a pistol and a rifle. Gallery Launch Gallery AP

The store's owner -- Kory Krause -- says multiple veteran staff members, who are trained to identify warning signs of potential mass shooter, spoke with Westman ... and none felt the killer was a threat.

Krause say

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