CONCORD, N.H. — The grandmother of an 11-year-old girl found dead in a river four years ago says the mystery surrounding the child’s death gets harder to deal with as the years drag on. Marcia Laro says she’s often asked if there have been any developments regarding her granddaughter Celina Cass, who was last seen hunched over a computer in her family’s apartment in West Stewartstown on July 25, 2011. “We can’t say anything because we don’t know anything,” she said Friday. Celina’s body was found a week later in the Connecticut River, after hundreds of police had searched in vain for days.