>> yellow starthistle is not a rare plant. it is an extremely common plant. so in any given area of -- especially of northern california, and at low elevations, yellow starthistle could be there. >> and with the other flora under the car, it shows that he's been offroad. so he could have picked it up anywhere. >> investigators agreed, so they turned their attention to the linear marks found on genna's thigh. >> the marks were made by a manmade object. they were clearly of a shape and appearance that would not be normally found in nature. >> while looking inside doug mouser's car, detective bosma thought he found what caused them. >> the hair on the back of your neck stands up and you're going, this could be one of the marks on her leg. >> but he didn't tell anyone about his discovery. instead, he sent photographs of genna's skin marks to gary