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Monday’s shooting at the Navy Yard in Washington, D.C. marks the fifth mass shooting since Newtown. MSNBC.com’s Richard Wolffe, Dave Cullen, author of “Columbine,” Heather McGhee from Demos and Mark Glaze from Mayors Against Illegal Guns join Alex Wagner to discuss the different factors that can play into the motivations of mass shooters.Sep.17.2013