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

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

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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

itoggle caption Larry Downing /Reuters /Landov

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

itoggle caption David Manning/Reuters /Landov