The Pact by Jodi Picoult

Until the phone calls came at three o'clock on a November morning, the Golds and their neighbors, the Hartes, had been inseparable. It was no surprise to anyone when their teenage children, Chris and Emily, began showing signs that their relationship was moving beyond that of lifelong friends. But now seventeen-year-old Emily is dead - shot with a gun her beloved and devoted Chris pilfered from his father's cabinet as part of an apparent suicide pact - leaving two devastated families stranded in the dark and dense predawn, desperate for answers about an unthinkable act and the children they never really knew.