SPOILERS for INTO THE WATER by Paula Hawkins

Lena's friend killed herself because she had been having an affair with her teacher. When her teacher ended it, then her friend threw herself in the river.  Nel was having an affair with the police officer, Sean Townsend.  In her research, for her book, Nel suspected that Sean's mother did not jump to her death.  In an effort to recreate his own mother's death at the river, Sean pushed Nel to her death.  His father admitted to killing Sean's mother and covered for Sean, saying he killed, Nel, too.

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