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Associated Press
FILE - This combination made from photos provided by the New York Police Department shows officers Rafael Ramos, left, and Wenjian Liu. Authorities say Ismaaiyl Brinsley, who vowed online to shoot two "pigs" in retaliation for the police chokehold death of Eric Garner, ambushed Ramos and Liu in a patrol car Saturday, Dec. 20, 2014, and fatally shot them in broad daylight before running to a subway station and killing himself. The killing of the officers highlighted shortcomings in the warning systems that were used. (AP Photo/New York Police Department, File)

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