Holly Bobo's brother, who saw her being abducted, and parents recall day she vanished

Clint Bobo was the last person to see the 20-year-old nursing student alive.

“It was really hard knowing that if they’re looking in the wrong direction … it’s just that feeling of, ‘What if we never know? What if we … never find Holly?’” Clint Bobo told ABC News' “20/20.” “I was just like, 'What if this is never solved?' I think that’s been my worst nightmare in this.”

Watch the full story on ABC News "20/20" on Friday, Sept. 29 at 10 p.m. ET

Holly Bobo was a 20-year-old nursing student when she disappeared from her home in Parsons, Tennessee, on April 13, 2011.

Early that morning, Clint, who was then 25-year-old college student working on his degree to become a social worker, said he got a call from his mother, Karen Bobo, saying a neighbor had heard a scream coming from their house. Karen asked him to check on his sister.

Clint Bobo said he then looked out the window and told his mother he could see a man dressed in camouflage, who he thought was his sister’s boyfriend Drew, walking off with her toward the woods behind their house.

“I knew it was Holly,” Clint Bobo said. “I never saw my sister’s face, and I also never saw his face… I expected Drew to be dressed in camouflage because Drew and I had talked the night before and he told me he was going turkey hunting that morning.”

As he watched the two figures, Clint said his sister “wasn’t stumbling” and was “walking on her own” toward the woods. He said he noticed the man was holding a black object, which he thought was a deer grunt, a device used in hunting.

“She didn’t appear to be hurt,” he said. “I was still very confused. … It still didn't enter my mind that Holly was being abducted.”

But Karen Bobo, who was already at work at a local elementary school where she taught second grade at the time, knew her daughter's boyfriend had gone out hunting early that morning and hadn’t been at their house.

“I said, ‘That’s not Drew. Get a gun and shoot him,’ and Clint said, ‘You want me to shoot Drew?’” Karen Bobo told "20/20." “And I think that’s when I hung up and called 911.”

Karen said she was so hysterical that a friend drove her home. When she got there, Karen said she saw a pool of blood in their carport, which would later be identified as Holly's, and she started running through the woods, calling her daughter’s name, but she was nowhere to be found.

Karen's husband, Dana Bobo, also raced home, and neighbors and law enforcement descended onto their home, canvassing the area for clues. Clint said suspicion quickly turned to him and police asked him to remove his shirt to check for scratches or anything else indicating he had gone into the woods with her.

“I was just shocked that they [law enforcement] are looking at me,” Clint Bobo said. “To me, it’s common sense if I’m here, and Holly’s somewhere else, then obviously … I have nothing to do with it.”

Authorities have also said they were suspicious of Clint for a while because they said he kept changing his story from what he saw that morning, but Clint claims he was just trying to remember as many additional details as possible.

“I tried to do everything that they asked, really, plus more,” he continued. “I tried to be as helpful as I could. I know that it is viewed as, probably to a lot of people, as changing my story. But I don't view it that way. I view that as me being compliant with law enforcement.”

Authorities eventually cleared Clint, and to this day, he denies having anything to do with his sister’s disappearance.

“We just kept Clint fairly close to us,” Karen Bobo said. “I did become very protective of him. But … I knew the truth would come out.”

Early in the investigation, brothers Zach and Dylan Adams, as well as two cousins, Shayne Austin and Jason Autry, came under suspicion, but at the time authorities said there was no evidence they had taken Bobo. The four men also had alibis during the time of the crime, according to investigators.

As it turns out, Karen Bobo said she taught Zach Adams and Autry when they were in grade school.

On March 5, 2014, Zach Adams was indicted on charges of felony first-degree murder and aggravated kidnapping. Autry was indicted on the same charges on April 29, 2014.

Some of Holly Bobo’s remains were found a few months later on Sept. 7, 2014, by two men who were hunting for ginseng a few miles from the Bobos' home. Her skull was found with what was believed to be a bullet hole in it.

Zach’s brother Dylan Adams, who was already in police custody on unrelated weapons charges, eventually told police that he, Zach, Austin and Autry were involved in Bobo's disappearance.

Dylan Adams was originally charged with evidence tampering, but eventually, he, his brother Zach and Autry all faced first-degree murder, aggravated kidnapping and aggravated rape charges in connection to Bobo’s case.

Shayne Austin had been granted immunity after cooperating with police, but then was found dead of an apparent suicide in February 2015. The other three men -- the Adams brothers and Autry -- pleaded not guilty to all charges.

Zach Adams’ trial started earlier this month. Dana, Karen and Clint Bobo all testified for the prosecution. On the stand, Karen fainted after she was asked to hold a few of her daughter’s things.

“I had seen pictures of Holly’s things,” she said. “But I didn’t realize that I was going to hold those things, touch those things. I just couldn’t take it. I couldn’t breathe.”

But even after that, the Bobos said they kept coming back to watch the trial.

“It was real hard,” Dana Bobo told "20/20." “But I listened and watched every bit of it… I wanted to hear what happened to our daughter.”

At the trial, Jason Autry, who had been offered federal immunity to testify for the prosecution, implicated Zach Adams in Bobo’s murder and said Zach told him he went to Bobo's house to teach Clint Bobo how to cook meth, which Clint denies, saying he has never been involved with drugs and had never met the four men outside of crossing paths with Dylan Adams once or twice.

After an 11-day trial, Zach Adams was found guilty on Sept. 22 of all counts -- murder, especially aggravated kidnapping and aggravated rape. He struck an agreement with prosecutors that allowed him to avoid the death penalty but to be sentenced to life in prison plus 50 years without the possibility of parole.

The Bobo family said they decided to agree with the prosecution to allow Zach Adams life to avoid the death penalty so that they can begin the healing process, but they say no amount of punishment is enough. At sentencing, Karen Bobo was given the chance to address the jurors and Zach Adams. She said she wanted him to see her face.

“I told him, ‘I 100 percent believe she fought, and fought hard, and then probably begged for her life,’” Karen Bobo said. “We showed him mercy, but I'm sure my daughter begged for her life, and he showed her no mercy.”

Jason Autry and Dylan Adams are facing the same charges. Prosecutors told ABC News that the next step in the case is an upcoming status conference for the two co-defendants to determine how the case is progressing.

Karen Bobo said there is “no doubt” in her mind that authorities have the right people in custody. She now wears the promise ring that her daughter’s boyfriend Drew had given her that was found in the woods.

“The world will miss out on one of the sweetest souls I have ever known in my life,” Karen Bobo said.

“I feel that still a part of us is missing,” Clint Bobo added. “And I feel it's always going to be that way.”