The acquaintance told authorities that Frank believed he could extort US$5 million (A$5.41 million) from Letterman by holding the boy and his nanny for 48 hours.
Frank's lawyer said at the time the remarks were "lighthearted conversation" and not a serious plan.
He struck a plea deal with prosecutors later that year that dropped the kidnapping charge. He pleaded guilty to theft for overcharging Letterman between US$1,000 and US$1,500 for the painting work he had done.
He also pleaded guilty to obstruction for misleading an investigator and to possessing illegally killed wildlife when authorities found a poached deer in his home.
He was sentenced to 10 years in prison and given a concurrent 10-year sentence when his probation was revoked for an unrelated intimidation offence.
In 2007, Frank escaped from the Montana State Prison ranch with another inmate and was apprehended six days later. He was caught after walking across the Mission Mountains barefoot and shirtless.
He was sentenced to 10 years in prison for escape. He became eligible for parole in May 2011.
- AP
