New York Police Department Commissioner William Bratton attends a press conference after witnessing police being retrained under new guidelines at the Police Academy on Dec. 4. Andrew Burton/Getty Images hide caption

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New York City Police Commissioner Bill Bratton speaks during an NYPD swearing-in ceremony in New York on Jan. 7. He confirmed to NPR today that there had been a work slowdown by officers in the weeks since two police officers were shot dead. He said the matter was being corrected. Seth Wenig/AP hide caption

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New York Police Department Chief of Detectives Robert Boyce said Sunday that a gunman told bystanders to follow him on Instagram, then shot and killed two police officers in Brooklyn on Saturday. Stephanie Keith/Reuters/Landov hide caption

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Thousands gathered on the National Mall last week to protest the deaths of Eric Garner and Michael Brown. Some law students say their involvement in the protests means their exams should be postponed. Saul Loeb/AFP/Getty Images hide caption

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Congressional staff members gather at the Capitol to raise awareness of the recent killings of black men by police officers, both of which did not result in grand jury indictments, on Dec. 11. They are joined by Rep. Elijah Cummings, D-Md. (second from left), and Senate Chaplain Barry Black (far right). J. Scott Applewhite/AP hide caption

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Melissa W. Green, right, and her daughter Reshae Green holds up their signs at Freedom Plaza during the "Justice for All" march and rally on Pennsylvania Avenue in Washington on Saturday. Jose Luis Magana/AP hide caption

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Black congressional staffers hold their hands up as they pose for a group photo during a walkout on on the steps of the U.S. Capitol Thursday, in a protest over the Eric Garner and Michael Brown cases. Alex Wong/Getty Images hide caption

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Members of the St. Louis Rams raise their arms to protest the grand jury decision in Ferguson, Mo., before a game last month. The players faced a backlash from St. Louis police and have been asked to apologize. L.G. Patterson/AP hide caption

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Protesters light a dumpster on fire, early Monday in Berkeley, Calif., as raucous demonstrations hit the streets of California for a second straight night in response to police killings in Missouri and New York. Taylor Nitta/AP hide caption

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A man carries a box of merchandise from a vandalized Radio Shack on Sunday during a march against the New York City grand jury decision to not indict a police officer in the death of Eric Garner, in Berkeley, Calif. Stephen Lam/Reuters/Landov hide caption

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