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Eleven police officers have been shot -- four of them fatally -- at a rally in the American city of Dallas against police violence. It is believed that the attack was carried out by several snipers. Rallies have been held in several US cities against the shooting dead of two black men by police officers in recent days in Minnesota and Louisiana. Britanny Packnett - an activist who sits on President Obama's Task Force for 21st Century Policing- gives her assessment of the increasing violence in America. ( Picture: Police cars sit on Main Street in Dallas following the sniper shooting during a protest. Credit: Laura Buckman /AFP/Getty Images)