Mohammed Al-Qawli, an educational consultant in Yemen, remains haunted by the day in January 2013 when he spent hours trying to collect the burned body parts of his brother Ali, a schoolteacher, and his cousin Salim, a student, both "accidentally" killed in a U.S. drone strike. Now Al-Qawli and rights group Reprieve are mounting a legal challenge against the U.K government for failing to investigate human rights violations by a British communications giant allegedly facilitating the killings of...