Split Second by Catherine Coulter

Split Second (An FBI Thriller) - A serial killer is preying on single women in bars.  When one would be victim gets away, DNA proves that the killer is related to another serial killer, now dead.  When an FBI sting goes bad, Dillon Savich becomes the focus of the killer.

Lucy Carlyle and Cooper McKnight are assisting Dillon and Lacey Sherlock.  But Lucy's dad dies, and Lucy learns that her grandfather was murdered.  Moving into her grandmother's house to find answers, what she finds presents more questions.  Also, she receives a ring that has mysterious qualities.