Briarpatch by Ross Thomas

A long-distance call from a Texas city on his birthday gives Benjamin Dill the news that his sister, it's her birthday, too, they were born exactly ten years apart, has died in a car bomb explosion. It's the chief of police calling, Felicity Dill worked for him; she was a homicide detective. Dill is there that night, the beginning of his dogged search for her killer. What he finds is no surprise to him, because Benjamin Dill is never surprised at what awful things people will do, but it's a real surprise to the reader.