The massacre left an imprint on Mitchem. After graduating from Providence College, she joined the Bureau of Alcohol, Tobacco, Firearms and Explosives. “Fearing for my friends and watching what was happening — you don’t forget things like that,” she told me. “I wanted to make a difference.”She started in the ATF’s Denver office as an industry operations investigator, the bureau’s term for inspectors who ensure that firearms dealers are conducting the required background checks on buyers and ma...

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