The F.B.I. doesn’t know her name, but they do know that she’s dead. She may have been named Marianne or Mary Anne. She may have been 18 or 19 when she was murdered. She may have been murdered in 1971 or 1972. Her body may be decaying somewhere in the empty western edges of South Florida’s swampland, but they aren’t sure, not really, about any of it.