According to Smart, Mitchell had whispered to her to stay quiet and that if she disobeyed he would kill her and her family.
Once they had left her home, Smart said Mitchell became worried the police would spot them and forced her to duck underneath a bush when a cop car drove past.
In the months following Smart's disappearance, her parents made multiple television appearances, pleading for their daughter's return. Hundreds of people scoured the region hunting for her.
Smart was found nine months later in March 2003 with Barzee and Mitchell, a self-proclaimed prophet who called himself "Emmanuel" and once had done work at Smart's home.
Both Barzee and Mitchell have been repeatedly declared mentally incompetent to stand trial for charges including kidnapping, sexual assault, burglary and conspiracy to commit kidnapping.
According to U.S. District Judge Dale Kimball, who determined Smart should be allowed to testify, Smart's inclusion in this latest review was necessary because Mitchell has refused to submit to any psychological evaluations or diagnostic tests, The Salt Lake Tribune reported.
For Smart's father, Ed Smart, there is no question about Mitchell's competency.
"He is the ultimate manipulator," Ed Smart told "Good Morning America." "He's manipulated the system to the nth degree. He is competent. He is a pedofile and he needs to be brought to trial.
"He hasn't been held accountable yet and I'm hopeful that this will really bring him to accountability," he said.
Last year, Smart told "GMA" that her life was "great," but that she still feared her alleged kidnappers.
"I think that if they were to be released, I think they would come back and they would try to come back after me," she said. "And I don't think ... that any child or any human should ever be in danger of having that happen again."
At the time, Smart expressed hesitation about testifying against the pair.
"I don't think I really want to, but I don't want them getting back out," she said. "I don't want them ever out because I really, really believe that they wouldn't stop."
ABC NEWS' Lee Ferran contributed to this report.