simpson. >> you were asked to attempt to determine approximate time of death for shirley carter, is thatrect? >> yes. >> were you able to do that to a reasonable degree of medical certainty? >> yes. >> reporter: wecht told the jury that shirley may have been killed two hours earlier than anyone had thought possible. if he was correct, then shirley was murdered around 9:00 a.m., when jason was seen on video clearly still at the granary. he could not have killed his mother. >> so when the state says to you, "all clues point to one conclusion," it's all the clues that the tunnel vision of law enforcement and one investigator in particular went after. >> reporter: the defense attorney said the tunnel-visioned investigator was special agent mark ludwick. now, he reluctantly became her star witness, with branstad rattling off the names of people who'd come up during the investigation. ludwick fumbled. >> you were not very aware of callie shin's involvement. >> that's probably correct, yes. >> not aware of jeremy morris? >> i did become aware of jason morris. i'm not sure of what point i was. >