Workers emerge from a building after a deadly shooting at the Washington Navy Yard on Monday. There are multiple injuries and deaths, and one gunman is dead. Police say they are searching for two other "potential" shooters. Alex Wong/Getty Images hide caption

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Alex Wong/Getty Images

Hadiya Pendleton. Courtesy of Pendleton family via the Chicago Tribune /MCT /Landov hide caption

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Courtesy of Pendleton family via the Chicago Tribune /MCT /Landov

William Spengler, in an undated image released by the Monroe County (N.Y.) Sheriff's Office. Reuters /Landov hide caption

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Reuters /Landov

Jacob Tyler Roberts. Clackamas County (Ore.) Sheriff's Department hide caption

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Clackamas County (Ore.) Sheriff's Department

In shock: The scene outside a Minneapolis business Thursday. A gunman killed four people, wounded four others and then killed himself. Richard Tsong-Taatarii/Minneapolis Star Tribune/MCT /Landov hide caption

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Richard Tsong-Taatarii/Minneapolis Star Tribune/MCT /Landov