Heather confirms in this letter, in so many words, that it was her that left the trail of bodies across the United States (Denton County, TX, Meeman State Park, TN, Indian Head, MD). She brags that no one, save JWJ, knows it was her and asks if he likes her work. 

She also mentions that she is waiting everyday at the obelisk (most likely Concord Point Lighthouse) for him to come and they will sail away on his boat. She talks about them not being of this world and this world is not meant for them. They would sail across the Styx. But then she speaks of burning her ID's and starting a new life with JWJ, "the only life I was meant to have".