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

itoggle caption Stephanie Keith/Reuters/Landov

President Obama announces the creation of a policing task force Dec. 1 as Philadelphia Police Commissioner Charles Ramsey (left) and George Mason University criminology professor Laurie Robinson look on. Mandel Ngan/AFP/Getty Images hide caption

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Michael Bell Sr. (center) and his family stand near one of the billboards they bought in a campaign to bring awareness to internal police investigations. Bell's son was shot and killed by police in Kenosha, Wis. Courtesy of the Bell family hide caption

itoggle caption Courtesy of the Bell family

Police wearing riot gear walk toward a man with his hands raised Aug. 11 in Ferguson, Mo. Renewed calls for police departments to hire more minorities have followed the shooting there of a black man by a white police officer. Jeff Roberson/AP hide caption

itoggle caption Jeff Roberson/AP

A police officer in Ferguson, Mo., stands guard as protests turned violent following Monday night's announcement of the grand jury's decision not to indict police officer Darren Wilson in the shooting death of 18-year-old Michael Brown. David Goldman/AP hide caption

itoggle caption David Goldman/AP

A police officer guards a closed street where protesters and looters rampaged businesses following the grand jury decision in the fatal shooting of Michael Brown, in Ferguson, Mo., on Tuesday. Jewel Samad/AFP/Getty Images hide caption

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Few police departments have required officers to wear body cameras, but that's changing after the events in Ferguson, Mo., this summer. Jim Mone/AP hide caption

itoggle caption Jim Mone/AP

Protesters gather outside the Albuquerque Police Department following the shooting deaths of James Boyd and others on March 25. The Justice Department accused the police of engaging in a pattern of excessive force. Rita Daniels /NPR hide caption

itoggle caption Rita Daniels /NPR

New York City police officers stand guard in Times Square earlier this month after a blog affiliated with the so-called Islamic State militants mentioned the area as a target for bombing. Andrew Burton/Getty Images hide caption

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In this image taken from video on Jan. 15, police officers Edward Sarama (from left) and Robert McGuire try to talk to officer Matt Dougherty, who is pretending to be mentally ill, during a training simulation at Montgomery County Emergency Service in Norristown, Pa. Michael Rubinkam/AP hide caption

itoggle caption Michael Rubinkam/AP