A girl was murdered and the investigator and her friends are trying to find the murderer but their hidden secrets stand in their way from finding the truth

How about:

When the body of a girl turns up [in some unique and specific way/place], her friends must put aside their differences/resolve old conflicts/confront hidden truths to get to the bottom of murder before someone else is murdered/something else terrible happens.

You could also add a midpoint reversal:

When the body of a girl turns up [in some unique and specific way/place], her friends turn on each other; but when a second body is found, they must put aside their differences/resolve old conflicts/confront hidden truths to uncover the murderer.

Love a good crime story!