The correct answer is Nathan .
"See the tire tracks on the right?" Amy whispered. "A car pulled in here from the low side. Then we see bicycle tracks starting from nowhere."
"Exactly. The killer prepared his crime ahead of time. He parked here, took a bike from his trunk or back seat and rode up the hill to attend the dinner party. He killed his victim in the garage, stuffed his body out of sight in the car and drove down here to stage his accident. He put a rock on the accelerator and slipped it into drive."
"Yes. After the murder, he retrieved the car and drove it the three miles back up to the house."
"The ground is too hard to show footprints. Footprints would be helpful."
"Helpful but unnecessary," Amy said. "The vital clue isn't here. It's up at the house, the very first thing we saw."
"The very first thing we saw were the cars in the driveway."
"Right. A single-lane driveway. Nathan had arrived first. Remember how Saul said he was surprised to see Nathan already there? That's because Nathan's Jeep wasn't in the driveway. Nathan drove up later, after the murder, and had to park behind Saul's Mercedes. That's the only way of explaining why Nathan's Jeep was behind the Mercedes instead of in front."