A woman cop uncovers scary evidence of an old murder and battles long-imbedded terrors to catch the serial killer before she becomes the next victim.

A female cop must stop a serial killer before she becomes his next victim.
(14 words)
The details in the original logline are for the story proper but not necessary for a logline.?

For a twist:
A female cop?must stop a serial killer who only preys on female cops before she become his next victim.
(20 words)

As Dkpough1 said the generic descriptions dilute the impact of the concept, best to be specific in your logline so as to set it apart from the rest in it's genre.

That said, what is it about this "cop turned victim" story that would make it interesting or different to many other similar stories? How will the stakes be personal or higher for this cop than others to make it a more compelling story?

Why would she become the next victim? Tie that in and it will make the story feel bigger and more together.