This gun store prides itself on stopping guns from ending up in the wrong hands. Then a customer was named as a mass shooter

Frontiersman Sports owner Kory Krause recounts Robin Westman's visit to his store during an interview, on September 4 in St. Louis Park, Minnesota. Mark Vancleave/AP

Five federal agents entered the gun store on the busy highway in central Minnesota that Chris Mercadante manages and gave him a serial number.

Unfamiliar, he flicked through the book that keeps a record of every gun that passes through Frontiersman Sports gun shop until he spotted it. It belonged to a revolver purchased by a customer just four days prior.

That customer had opened fire on Annunciation Catholic School in Minneapolis just hours earlier, authorities say, firing through the windows at the pews, killing two children and wounding 18 other people.

Worry had been stirring within 60-year-old Mercadante even befo