The Watchman by Robert Crais

Larkin Conner Barkley lives like the City of Angels is hers for the taking. Young and staggeringly rich, she speeds through the city during its loneliest hours, blowing through red after red in her Aston Martin as if running for her life. Until out of nowhere a car appears, and with it the metal-on-metal explosion of a terrible accident. Dazed, Larkin attempts to help the other victims. And finds herself the sole witness in a secret federal investigation.