Murder in Greenwich (2002 TV movie)
This is the story of Martha Moxley, a 15-year-old girl murdered in Greenwich in the 1970s. Her murder went unsolved for 25 years.
From IMDb 2:
In 1974, teenager Martha Moxley moves to the high-class area of Belle Haven, Greenwich, Connecticut. On Mischief Night, eve of Halloween, she was murdered in the backyard of her house, and her murder remained unsolved. Twenty-two years later, writer Mark Fuhrman, a former LA detective who had fallen into disgrace for perjury in the O.J. Simpson trial, decided to investigate the case with his partner, Stephen Weeks, with the purpose of writing a book. The locals did not welcome them, but, with the support of retired detective Steve Carroll, who was in charge of the investigation in the 1970's, they discovered the criminal and a net of power and money that covered the murder.
They do have Martha narrating.
EDIT: This is from an IMDb review:
Martha Moxley, whom they had tell the story in the movie, was bludgeoned to death by her violent troubled neighbor Michael Skakel. Michael did this out of jealousy of his brother, Tommy, when Martha rejected him and took Tommy instead.