Jasper Spires boarded the Red Line Metro train at Rhode Island Avenue shortly before 1 p.m. Saturday, joining passengers from the District and elsewhere headed to various Fourth of July festivities, among them the Foo Fighters concert at RFK Stadium. As the train rumbled toward its next stop, at NoMa-Gallaudet, a three-minute ride, D.C. police said, the 18-year-old Spires — who may have been high on synthetic drugs — tried to grab a cellphone tucked into the waistband of a recent American University graduate headed to a gathering with friends. The two struggled, police said, and the terror began.