I read this book not too long ago I don't think it's that old. I think it's set in michigan or wisconsin
it starts of with a woman waking up in a car with a small child that calls her mommy she has amnesia and can't remeber what happened her clothes are bloody and she has crashed the car they are driving in they are rescued by a woman who takes them to her home (she lives in a lighthouse and trains rescue dogs) because the first woman is afraid to go to the cops (don't know why) she thinks her name is elaina (I think) because it's engraved on a necklace she wears turns out it's the name of her old ballet instructer
then it skips to the detective hunting down a serial killer that just presumably killed the detectives wife and child the killer is leaving GPS coordinates to the bodies posted online
turns out the woman is his wife (they're getting divorced)and the child that they have been raising is her neice that was left with them by her drug-addict sister who later comes back in the book and tries to take the child from them

in the end the killer is killed by swans

please help!