The whole movie is flawed.
After the kid reports that he saw his stepfather kill a man, the detective comes back into the room and tells everyone that the garage didn't find any evidence that implicates that a murder did happen in the car.
I mean are we in the 1800's? all the Vaughn's character did was clean the blood off the seat with some tissue and cleaning fluid. Surely that erased all the blood! if they went through the trouble of going to the area where the murder happened why the hell would they not use that forensic tool that finds blood even after it was cleaned (can't believe I forgot the term/name)