4th of July by James Patterson (2005) Little Brown & Co.

The Lieutenant chases a couple of teens joyriding away from a murder scene, they crash the car and Lyndsey and her partner holster their guns to help the kids from the burning vehicle, but the kids pull their own guns and shoot them.  From the ground, Lyndsey shoots back, killing the girl and permanently disabling the boy.  The result is a wrongful death lawsuit against Lyndsey and the police department.  While waiting for trial in Half Moon Bay, a series of murders occurs there.  Lyndsey is torn between waiting for a trail and wanting to solve a case out of her jurisdiction.