Sanford, Fla., police Officer Timothy Smith holds up the gun that was used to kill Trayvon Martin, while testifying on June 28, 2013, the 15th day of George Zimmerman's murder trial. Zimmerman has listed the gun for sale online. Pool/Getty Images hide caption

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Pool/Getty Images

Dr. Martin Luther King Jr., marches with other civil rights protesters during the 1963 March on Washington. AP hide caption

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AP

Classroom Reflections On America's Race Relations

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A booking photo provided by the Seminole County Public Affairs shows George Zimmerman on Saturday, Jan. 10, 2015. In the latest in a string of run-ins with the law in the past two years, Zimmerman has been charged with aggravated assault. AP hide caption

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President Obama tackled race head-on in his first on-camera response to George Zimmerman's acquittal in the shooting death of Florida teenager Trayvon Martin. Carolyn Kaster/AP hide caption

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Carolyn Kaster/AP

President Obama during his appearance at the White House on Friday. Saul Loeb /AFP/Getty Images hide caption

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Saul Loeb /AFP/Getty Images

How President Obama 'Showed His Brother Card'

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President Obama at the White House on Friday, as he spoke about the death of Trayvon Martin and the national discussion that the case has generated. Larry Downing /Reuters /Landov hide caption

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Larry Downing /Reuters /Landov

President Obama's July 19, 2013, comments on Trayvon Martin and race relations

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Attorney General Eric Holder speaks at the annual convention of the NAACP in Orlando, Fla., on Tuesday. Holder told the convention that "Stand Your Ground" laws that have been adopted in 30 states should be reconsidered. David Manning/Reuters /Landov hide caption

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David Manning/Reuters /Landov