The Long Body — Helen McCloy

A prominent American diplomat falls over a cliff to his death. The death is accepted as an accident, but could it have been suicide - or even murder? His widow finds a locked drawer in his desk and in it a file with a woman's name on it - but the file is empty. Circumstances lead her to an elderly man bearing the same name, but he has a stroke and can neither speak nor write. And then she sees the car headlights coming at her, fast, at night, through an impenetrable mist ...