Transcript for Man goes on trial for nursing student Holly Bobo's murder: Part 4
Reporter: This summer holly Bobo's cousin, country singer Whitney Duncan released a song about holly, called better place. ??? I bet you don't miss this town ??? The song was kind of me talking to her. ??? I pray and I pray and ask god why ??? so I really wanted to show some clips of holly, from when she was a baby and just of her growing up through the years. But I know she's watching down and she sees everything. ??? Better place ??? God bless the United States, the state of Tennessee, the county and this honorable court. Reporter: Earlier this month, Zach Adams death penalty murder trial gets underway in an ornate, chandeliered courtroom in Hardin county, Tennessee, Reporter: Zach Adams, standing alone as the face of evil. His husky appearance strikingly different from the gaunt man seen in preliminary hearings two years earlier. Prosecutor Paul Hagerman delivers his opening statement in a voice so quiet, many strain to hear. He took her. He raped her. He killed her. And he almost got away with it. Reporter: Jennifer Thompson is Zach Adams' defense attorney. He did not, as a matter of fact, know holly Bobo, and had never even laid eyes on her. Reporter: When holly's mother, a school teacher, testifies, a surreal moment. She knows her daughter's alleged killer. She taught Zack Adams in the 4th grade. She's keeps her composure until she's asked to identify some of holly's belongings. Is this her car key. Yes, ma'am? Let's take a minute. Let the jury file out, please. Whoa. Get a nurse. Reporter: Now prosecutors introduce a recent discovery, what they consider the murder weapon, a pistol allegedly once owned by shayne Austin. It had been recovered just before the trial was set to begin. Finding the gun came out of nowhere. Nobody could ever find a gun. Reporter: If it's the smoking gun it hasn't smoked in a while, the pistol was found under water in a drainage ditch. One expert testifies based on the size of bullet hole it could be the murder weapon. But no expert proves it actually is. And Dylan Adams is also nowhere to be seen, not on the witness stand; not in that videotaped interrogation. That's because the state no longer needs him, because prosecutors have found a real rock star witness, none other than Jason Autry himself! Be seated. State your first and last name and spell it for the court please. Jason Autry. They had no physical evidence other than a skull, and a gun. So Jason Autry is their best hope at a conviction. When Jason Autry came along, it's what the state really needed to try to get a conviction of somebody, and I think the state really needed to convict somebody. Reporter: For years, Jason Autry had been proclaiming his innocence to anyone who'd listen. I want to say one thing, I'm innocent of these charges. Right hand before god, I'm innocent sir. Reporter: But this time with his right hand before god, it's a different story. Right there is the location where she was killed. He said the morning she was taken Zack Adams asked him to get rid of a dead body in the back of his pickup truck. He said I need you to help me bury this body. Reporter: He says according to Zach Adams the body in the blanket was holly Bobo. Autry says he and Zach drove the body down back country roads to a spot on the Tennessee river under the interstate 40 bridge. Cell phone records show they were both in that area that morning and this graffiti suggests Zach had been there before. He says they laid the body in the blanket on the ground. At that time I see the foot move, a movement and a sound of distress that sounded like pin come out of her voice, come out of her, come from the blanket. I told him, I said, "This Is still alive." Reporter: He says he kept a lookout and Zach fetched a pistol from the truck. At that time boom, the gun went off. It sounded like boom, boom, boom underneath that bridge. It was just one shot, but it echoed underneath that bridge all the way down that damn river bottom. Birds went everywhere, just all up from under that bridge. It was just dead silence for just a second. Reporter: He says they load holly's dead body still in the quilt back onto the truck. Shut the tailgate and tear out of there like wild Indians. Jason Autry is a star witness like no other. Just details his entire day on April 13th like it was yesterday. Reporter: Autry claims that was the extent of his involvement. He says he eventually learned from Zach and Dylan Adams that they and shayne Austin had kidnapped holly earlier that morning and sexually assaulted her in an old barn. After shooting her under the bridge, he says Zach disposed of her body. Did Jason Autry's story on the stand sound plausible to you? It did. Every part of it? It did, and it matched. It matched what the prosecutors had also. Reporter: Zack Adams' defense attorney says it's a pack of lies. I think Jason Autry's gonna be rewarded for his story. I think he's gonna be released. I think he'll be released fairly soon. Reporter: She points out phone records show Autry takes time out from allegedly participating in this murder to chat on the phone with his girlfriend, and his mother. In the middle of all this, you were able to take a telephone call from your mother Shirley king that morning, at 9:42, weren't you? If your records reflect that, that's true. Reporter: But prosecutors point to their own phone records, bringing on a tbi expert who tells the jury that Autry's story of meeting up with Zach Adams before traveling to the Tennessee river is backed up by the movements of their cellphones. But the prosecution isn't finished. They call to the witness stand a rogues gallery of walking, talking, testifying mugshots. Cons and ex-cons who say they heard Zach Adams bragging about what he had done to holly Bobo. He said, "I'll kill you like I did holly Bobo." I couldn't have picked a prettier -- he made the comment that, "It sure was fun." Reporter: The prosecution's case leaves some nagging questions unanswered. Which of the suspects took holly from her home? Why was she targeted? And, what happened to her before she was wrapped in the quilt in the back of the pickup? That's what leads the defense, and the original invest gator to say the wrong man is on trial. And now Terry britt, the original suspect, is about to take the stand!
This transcript has been automatically generated and may not be 100% accurate.