Campbell was smoking a cigarette in the parking lot -- about three parking spaces away from Motz's car at the time of Melissa's death. From 46 feet away, Campbell said he did not hear a gunshot.
"Even with the windows up I still would've heard ... something," Campbell said. "It was very quiet outside. I was only standing there, it was late at night. ... I've thought about it over and over, many times. And I just feel like I would've heard a gunshot."
Melissa's parents sued their son-in-law for wrongful death. After a trial in 2007, a jury found Motz had caused Melissa's death, although they didn't have to decide whether she committed suicide or he killed her.
Members of the jury told "20/20" that the witness's account that the fatal gunshot was not heard raised eyebrows.
"That bothered us a lot," a juror said. "It led me to believe that the shot didn't happen ... at the apartment [parking lot] ... it happened before they got there."
"20/20" asked Gast if the coroner's office had any plans to recreate the incident that night to see if the gunshot could be heard from that distance. She said that was "certainly a possibility."
"20/20" decided to put a significant element of Jimmy Motz's story to the test, hiring forensic firearms analyst Mike Haag to re-create the scene of Feb. 16, 2001. Haag couldn't fire a gun in a Rock Hill, S.C., apartment complex in the middle of the night, so we came to the Tonto National Forest, outside Scottsdale, Ariz.
"20/20" found the make, model and year Ford Thunderbird that Jimmy and Melissa Motz rode in the night of her death.
Haag bought the exact gun -- a .32 H&R magnum revolver -- and loaded it with the exact type of ammunition she used to supposedly commit suicide. He prepared blocks of gelatin and animal skin to simulate human tissue.
"It's the correct density and elasticity, and many different properties, to simulate as best we can how deep bullets should go in typical tissue," Haag said.
The tissue stimulant was duct-taped to the head rest, approximately where Melissa's head would have been inside the vehicle. A piece of a Kevlar bullet-resistant vest was placed behind the back of the block to catch the bullet in case it completely penetrated the tissue, Haag said.
Haag measured off the exact distance witness Chris Campbell says he was standing from Melissa Motz in the parking lot that night and set up a sensitive sound meter, 46 feet away.
With the same car, the same gun and similar circumstances, Haag said the experiment is "as close as we're ever going to know" if the gunshot would have been audible.
If there was no sound from a gun fired inside this car, it would add credibility to Jimmy Motz's story that Melissa shot herself. If the shot was audible, it would raise even more questions about where Melissa was shot, and by whom.