How does Marlena die in the novel, Marlena?

A key element of the book as a whole is how many of the specific consequences associated with Marlena's death remain unexplained. There are a number of "likelies" and "possibilities" associated with what happens to her - her death was "likely" drug related, "possibly" connected to her relationship with Bolt, and very "possibly" an accident. Ultimately, though, the causes of her death remain as unknown, to Cat and to the reader, as is so much of her life.