“The answer is found in Chapter Six, where you will recall that Susan, Paul, Harry, Jonah and Susan’s investigator meet at Paul’s office. This is immediately after Paul has had his confrontational meeting with Zolanda Suade at her office. Paul has just picked Jonah up at the docks and Jonah is covered in fish blood from his catch of the marlin that he leaves hanging on the docks. In Chapter Six he has not had time to change, but is in Paul’s office in his fishing togs. He is also smoking a cigar and offers them all around, including one to Susan. She takes it. At the end of this meeting, who does Jonah leave with? He leaves with Susan in her county car to go to Susan’s office in order to prepare documents to compel Suade to disclose the location of the child. The evidentiary doctrine involved here is called “Transference”. Trace evidence such as fish scales, and even traces of dried or drying blood would easily be transferred to the upholstery of Susan’s car as Jonah sat in the passenger seat.
Later that evening when the killing occurred, you will recall that the evidence indicated that Suade had similarly sat in the passenger seat of the killer’s car — Susan’s car, that she was shot in the vehicle and her body dumped in the parking lot behind her office. The ashtray containing cigar ash that Jonah had smoked earlier in the day was similarly dumped on the body — why? Because Suade smoked cigarettes and Susan did not want any evidence in her car linking herself to the victim. She didn’t realize that she was inadvertently incriminating Jonah by dumping the ash.
My editor and I actually debated whether we should include an explanation of the above in the final chapter of the book, but felt it wasn’t necessary as it smacked too much of a Hercule Poirot mystery. Perhaps we were wrong.”