Amnesty International is calling on the U.S. Justice Department to investigate the violent police crackdown against Native American water protectors and their allies fighting the $3.8 billion Dakota Access pipeline at Standing Rock in North Dakota. The call comes after Amnesty sent a delegation of human rights observers to witness the police crackdown, which has included police using rubber bullets, tear gas, concussion grenades, sound cannons, water cannons in subfreezing temperatures and other military-style weapons that have injured hundreds of people.