Stingrays are routinely used to target suspects in drug and other criminal investigations, but activists also believe the devices were used during protests against the Dakota Access pipeline, and against Black Lives Matter protesters over the last three months. The Justice Department requires federal agents to obtain a probable cause warrant to use the technology in criminal cases, but there is a carve-out for national security. Given that President Donald Trump has referred to protesters as “terrorists,” and that paramilitary-style officers from the Department of Homeland Security have been deployed to the streets of Portland, Oregon, it’s conceivable that surveillance...