In the Huntleys' eyes, there was one suspect: Jimmy Motz.
Motz said Melissa had been prescribed an antidepressant and claimed she'd attempted suicide a few months before, but he kept it a secret until her death.
"You've got someone who suffers from depression. You've got someone maybe who was tempted to commit suicide before. You've got all those factors," said Motz's lawyer, James Boyd. "Now is it logical that she'd do it, no. Is it consistent with everything that happened, that she would do that? I think yes."
But police also began to doubt Motz's suicide story. According to a forensics exam, police said the angle of Melissa's wound suggested she may have been shot by someone next to her.
"If she had fired the gun, it would have been an unnatural way that she would have had to have held the gun," Cabaniss said, noting that Melissa was right-handed, but gun residue was found on her left hand. "It would have been very easy and more natural to occur from someone from the driver's side of the vehicle."
Given the police's suspicions and mounting evidence, the Huntleys were shocked when the local prosecutor at the time, Tommy Pope, told them he was not going to prosecute Jimmy Motz, believing there was insufficient evidence for a conviction.
"The prosecutor's job is to try to see that justice is done for the victims, as well as the defendants. And in this particular case, it did not warrant going forward," Pope told ABC News' Jim Avila.
"20/20" filed an open records request and recently gained access to the evidence and police records related to Melissa Huntley Motz's death.
Along with the gun that fired the fatal shot, and the misshapen bullet retrieved from Melissa's head, was a three-page document -- the case summary of the Rock Hill Police Department's investigation into Melissa's death. It showed that detectives discovered Motz had a long criminal record, including some dark and violent events in his past.
Motz's two previous marriages ended in divorce, and the grounds in both cases were physical cruelty. He had also been convicted of criminal domestic violence against one of his ex-wives, Sandy, who gave police a sworn statement detailing how her husband would fly into a fit of rage and threaten her by placing a gun under her chin.
"My first reaction was that he killed her," Sandy Motz said in a police statement on the circumstances of Melissa Motz's death. "I was not surprised, because I [had] been in her position so many times before."
Also, because of his history of criminal domestic violence, Motz wasn't legally allowed to buy or even possess a gun. The revolver had been purchased by his 80-year-old mother five years earlier.
The police investigation summary ended with the following stunning sentence: "Considering the wound, the path the projectile traveled, James Motz's past history, the witness not hearing a gunshot from less than 50 feet away, along with the pinch marks on Melissa's palm of her left hand, indications are that James Motz possibly placed the gun under her chin and pulled the trigger while they were both seated inside the car shortly before arriving at their apartment."