SPOILERS for The Lady in the Lake by Laura Lippman

I thought it was Ferdie who had killed the little girl and Cleo.  I was wrong. 

It was the pet store clerk and his mother who killed the little girl.  Cleo was not the Lady in the Lake. It was her roommate who had OD'd. Her lover's friends had paid her friend at the night club to kill her, but they were friends, so they concocted this scheme to make it look like she was the body in the lake. Cleo had shown up at the hospital where Maddie was a patient after being stabbed by the pet store clerk's mother.

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