It was supposed to be a good day — two teenage friends out on a lark, a Friday afternoon trip to the mall. Walio Mohamed, who was 17, slung her book bag, stuffed with her high school planner and a copy of Jack London’s “Call of the Wild,” over her shoulder as she and Destinee Mangum, then 16, boarded a MAX commuter train in Portland, Oregon. The two laughed joyfully, heads leaning into each other — Mangum sporting a bejeweled cloth headband; Mohamed wearing a black cotton hijab.