Crown and defence lawyers are asking that a high-risk sex offender who admitted to killing a Surrey teen spend 17 years in prison before being eligible for parole.

Raymond Lee Caissie, 46, pleaded guilty a week ago to second-degree murder in the death of Serena Vermeersch.

The 17-year-old's body was found by railway tracks in Newton in September 2014, hours after she was reported missing by her mother. Caissie was charged a week later.

Investigators say the murder was a random attack and a crime of opportunity.

The charge carries an automatic life sentence, but Thursday's court proceedings will determine how long Caissie will have to serve before he's eligible to apply for parole. That time period could set between 10 and 25 years.

At a sentencing hearing Thursday, Crown Counsel Colleen Stewart revealed shocking details of the murder in a joint statement of admissions.

The court heard that Caissie followed the teen off a transit bus and forced her into a secluded area.

The teen was able to slash her attacker's neck with an exacto knife she was carrying in her purse for protection, but he slapped the weapon away and choked her.

Caissie decided to kill Vermeersch, who was rendered unconscious, to "finish her off" so she wouldn't tell anyone, according to lawyers.

He used a piece of his t-shirt to strangle the teen before taking her purse and fleeing.

The purse and t-shirt were discarded a short distance away and he threw the knife into bushes before a friend picked him up.

Vermeersch died of asphyxiation, and Caissie required 22 stitches for the injuries to his throat.

“I won’t forget Serena until the end of my life and other people won’t either,” her mother, Rumiko Vermeersch, said in a victim impact statement read by the Crown Thursday.

“I failed her. I couldn’t protect her,” she said, adding that she keeps asking herself why she didn’t pick her daughter up from the bus stop that night.

The family was struck by even more tragedy when Vermeersch’s father, Kevin, died two months later, following a car crash that had left him in a coma.

Caissie, whose sentence will be handed down on Oct. 20, has a long criminal history.

In 1991, he kidnapped and sexually assaulted a 21-year-old woman who was working at Abbottsford’s Tretheway House museum. After forcing her to withdraw her daily limit from an ATM, he raped her again and left her in a field near the U.S. border, gagged and tied to a tree.

He was sentenced to 22 years in prison, a sentence that ended the year before Vermeersch was killed.

Caissie was the subject of a 2013 public warning by RCMP. Mounties said he had "a varied pattern of offending, having offended both violently and sexually, in both an opportunistic and impulsive manner.”

A probation order in July 2013 required him to regularly report to BC Corrections, avoid drugs, and avoid contact with his victims.

He was under surveillance by the RCMP, but court records show he stole from a Winners department store in Surrey two months later.

He failed to report to his probation officer twice in 2013, on Nov. 5, and again nine days later. Almost a week later authorities issued a warrant for his arrest.

Caissie was given three months in custody and three years’ probation for the breach.

Documents from the National Parole Board in 2014 deemed the convicted rapist a high-risk to reoffend because of intense aggression and prison violence.

But government lawyers didn’t ask for electronic monitoring or a curfew when he was released, a decision that was questioned after the sex offender was charged in Vermeersch’s homicide.

With files from CTV Vancouver’s Nafeesa Karim