A jury has found Garry Handlen guilty of first-degree murder more than 40 years after the death of 12-year-old Monica Jack.

During the 11-week trial, the court heard Handlen's alleged confession to an undercover RCMP officer who heard the man say he abducted, sexually assaulted and strangled Jack, who disappeared in May of 1978 while riding her bike along a highway in Merritt.

Handlen, who pleaded not guilty to the charge, was part of a sting operation in 2014 during which he told a supposed crime boss he grabbed the victim, threw her bike in a lake, and forced her into the bathroom of his camper before driving to the spot where he killed her, burned her clothes and left her body.

Some of Jack's remains, including her skull, were found in the area in 1995.

The killer's legal team maintained the hidden-camera confession was coerced, while prosecutors argued Handlen had no motivation to admit to a crime he didn't commit and felt relieved after he'd shared information about his involvement in Jack's death.

"It's a weight off my shoulder now, I've told you. So I'm not the only one that knows now," he tells undercover cop.

The nine-month Mr. Big operation involved a made-up crime group that paid Handlen nearly $12,000 and offered him a job.

In a bid to get Handlen, then 67, to open up about the details on the crime, the undercover officer can be heard telling him in the recording that Mounties had forensic evidence tying him to Jack's death.

"The bottom line is, they got people that saw you and they got your DNA," the officer says in the video. "That's not good, Garry."

At trial, the victim's mother gave the court of an emotional account of the last time she saw her daughter alive. Madeline Lanaro said she was driving home with her other children when she saw Jack on the highway.

"I honked and the kids yelled out, 'Do you want a ride? And she said 'No,'" Lanaro told the court.

Nine men and three women started their deliberations Tuesday and had reached their verdict by Thursday afternoon.

Cheers broke out in the courtroom as the verdict was announced. People in the gallery could be heard calling out "thank you" as jurors left the courtroom.

Members of Jacks' family could be seen crying and embracing each other.

One woman who says she's the victim of a 1978 rape by Handlen, for which he was convicted, was also at court.

"This is justice for all of the women this man may have hurt in his lifetime," she told CTV. "Good day in court today, you know, and I'm grateful I'm alive."

The case will return to court on Jan. 28 for sentencing. First-degree murder convictions carry an automatic life sentence with no chance of parole for 25 years.

Jack's family will have a chance to make victim impact statements during the sentencing hearing.

Handlen's defence team did not comment on the verdict Thursday.

With files from CTV Vancouver's Maria Weisgarber and The Canadian Press