A violent offender with a long criminal history who attacked an Edmonton jogger four years ago has been declared a dangerous offender.

Ashton Natomagan was sentenced Tuesday to a 20-year prison term for sexual assault with a weapon, robbery, kidnapping and choking.

The sentence will be followed by 10 years of close community supervision. With credit for time already served, Natomagan's sentence will be reduced to 16 years and eight months.

Natomagan, 38, has spent most of his adult life behind bars. His history of sexual offences spans 13 years and involves three victims.

Over that time span, he sexually assaulted two girls aged 11 and 16 and a 37-year-old married mother of two.

His most recent crime was committed in April 2015, when he attacked a stranger. The victim, whose name is protected by a court-ordered publication ban, was jogging in central Edmonton when Natomagan grabbed her, choked her and dragged her into an alley. He threatened to slash her with a box cutter.

At least one of his other victims was randomly targeted.

"Plainly, there is a pattern of repetitive, violent, criminal behaviour," Edmonton Court of Queen's Bench Justice Terry Clackson wrote in his 31-page sentencing decision. "By all accounts, Mr. Natomagan remains a high risk to sexually re-offend."

Clackson said that given the risk Natomagan presents to the public, he will likely serve his full sentence. But even that, he said, may not be enough to protect potential future victims.

A large portion of Clackson's written decision was devoted to what he described as "the impact of Canada's mistreatment of Indigenous persons."

"The potential benefits of Indigenous methods for healing, crime prevention and personal development must be considered," Clackson wrote. "His life and his personality have been directly impacted by Canada's mistreatment of his community."

'The cycle needs to be broken'

Clackson referred extensively to what he described as "powerful evidence" given by forensic psychologist Leslie Block.

Block concluded that the current treatments offered by Canada's correctional system will not help Natomagan, and could make things worse. The judge agreed.

Court of Queen's Bench Justice Terry Clackson (Court of Queen's Bench of Alberta)

"The problem for Mr. Natomagan is that despite the programming and his efforts, he remains virtually the same," Clackson wrote. "His life is one long episode of violence. From childhood to prison, violence and substance abuse are the common denominators."

Block proposed an innovative approach to healing Natomagan once he has served his sentence, designed to break down his current values and beliefs and replace them with traditional Indigenous values and spirituality.

The "completely new paradigm," the decision noted, would require the extensive involvement of one or more Indigenous elders to direct and monitor Natomagan as he spends two years in isolation in a remote area, learning about survival, self-reliance and self-awareness.

It would also require Natomagan to make a commitment to sobriety.

Block suggested the first period of isolation should be followed by reintegration into society under the control of an elder, followed by a shorter period of isolation.

Clackson said he was moved by the psychologist's proposal.

"I am convinced that the program he described would benefit Mr. Natomagan and many more of our Indigenous citizens who presently languish in our jails," he wrote.

The judge noted that he does not have the authority to compel that type of program, which currently does not exist. But he called it a reasonable expectation that over the next 17 years, governments will come up with a new way to deal with Indigenous offenders.

"Decades of unconscionable poverty, abuse and incarceration mark the plight of many Indigenous persons," Clackson wrote. "The cycle needs to be broken.

"Continuing as we have in the corrections field, where true successes are seemingly rare, seems senseless."