snipped from link below:
As it stands, to prove the kidnapping of a victim who is 14 or older, (District Attorney) Cooper said, he'd have to prove that Elizabeth was falsely imprisoned, unlawfully removed or had her freedom restricted.
Further, to prove that Elizabeth was unlawfully removed, he'd need to prove to a jury that Cummins employed "force, coersion, fraud or something to that effect," the prosecutor said.
"What we run into here, of course, is this child is 15 and, according to reports, at least initially, she left of her own free will," he said.
The issue was especially concerning at the outset of the investigation, Cooper said. Cummins was charged only with sexual contact with a minor by an authority figure, a misdemeanor, and investigators worried that if police stopped the pair out of state, they'd be released because authorities couldn't detain them, let alone extradite Cummins, on a misdemeanor warrant.
Cooper ultimately felt comfortable adding the aggravated kidnapping charge after deciding that Cummins allegedly groomed his victim and was armed, the latter being a prerequisite for aggravated kidnapping.
"Under current law, it's really going to depend what the testimony of Ms. Thomas is," Cooper said, explaining that if she claims she left on her own fruition, the defense will argue Cummins is not guilty of kidnapping.
Cooper will then have to introduce circumstantial evidence that Elizabeth was coerced, and to be clear, the district attorney is confident the communications between Elizabeth and Cummins demonstrate "he was definitely trying to influence her in his favor," he said.
http://www.cnn.com/2017/03/28/us/tennessee-teacher-manhunt-state-kidnapping-law/