TORONTO - A jealous teenage girl who used sexual taunts in reams of online and text messages to orchestrate the killing of 14-year-old Stefanie Rengel, a girl she had never met but considered a rival, was convicted Friday of first-degree murder.

The victim's family made a plea for the court to send a message that murder has serious consequences regardless of age. The Crown said it will seek an adult sentence for the 17-year-old girl, known only as M.T.

Rengel was lured from her home on New Year's Day 2008, stabbed six times and left to die in the snow. The crime was allegedly carried out by M.T.'s boyfriend, who stands trial in the fall.

As the jury returned their guilty verdict a few muted cries of shock could be heard in the courtroom and both the Rengels and the girl's family were in tears.M.T.'s younger brother was the most visibly affected by the verdict, doubled over with his hands over his face and crying.

The Rengel family expressed mixed emotions.

"As a family we were relieved to hear the verdict but admit it is very difficult to be happy when so many people's lives have been ruined -- not only ours, but surely the accused family's as well," they said in a statement read by Crown attorney Robin Flumerfelt.

"We hope that at sentencing there is some justice for Stefanie and a clear message is sent that murder at any age has serious consequences."

M.T., a bespectacled, plump girl with mousy brown hair was unemotional in police interviews after Rengel's death and had been stoic throughout the trial. But shortly after the verdict was read Friday she began to cry. Her shoulders were shaking with sobs and she wiped tears away with the sleeve of her blue cardigan.

For a youth under 16 sentenced as an adult, first-degree murder carries a life sentence with no chance of parole for five to seven years.For a youth offender, the sentence is a maximum of 10 years, which includes four years served in the community.

The jury deliberated for about 21 hours over three days before reaching the guilty verdict.

Defence lawyer Marshall Sack, who had stressed the culpability of M.T.'s boyfriend during his closing statements, spoke briefly outside court.

"I'm simply going to say that a violent and foolish young teenager has destroyed the lives of two young girls -- poor Stefanie Rengel and my client," he said.

"And the tragedy of it is absolutely overwhelming."

Toronto police Det.-Sgt. Steve Ryan, who investigated the case, said the crime itself and who committed it was almost unthinkable, yet the repercussions don't end with Rengel's life being tragically cut short.

"(I'm) relieved yet sad -- sad because there's two families that are now destroyed," he said outside the court.

Ryan said the age of the culprits as well as the level of planning made this an unusual case.

"I found it very hard to believe that a 15-year-old girl was able to orchestrate that crime."

The boy, who can only be identified as D.B. and now 19 years old, faces a first-degree murder trial in the fall.

M.T.'s jealousy over Rengel, who had been briefly involved with D.B., consumed her and led her to urge him repeatedly to kill her, so that the younger girl she had never met wouldn't interfere in her relationship, the Crown had said.

While the girl was not accused of actually ending Rengel's life, prosecutors relied on a well-established legal principle that counselling others to kill makes a person guilty of murder.

M.T. dangled the promise of sex in front of D.B., saying he would have to kill Rengel first, the Crown said, referencing a huge volume of text and instant messages between the young couple.

The defence did not call any evidence but argued in closing statements that the jury shouldn't draw murderous intent from those messages.

As M.T. and D.B. were under 18 at the time and cannot be identified under the Youth Criminal Justice Act.

The Crown entered into evidence more than 30,000 pages of instant messaging transcripts between the girl and her boyfriend, including conversations apparently detailing their plans for Rengel.

In one chilling instant-message exchange in October 2007, the convicted girl told her boyfriend she wanted Rengel dead.

The boyfriend, who had a brief romantic encounter with Rengel when she was 12, wrote: "What about Stef," according to transcripts filed with the court.

"Bang, bang," the convicted girl responded.

He then wrote: "I need a bang bang first ... I wanna bang you."

She replied: "I want her dead ... lol we've been through this ... If it takes more than a week then we're just going to be friends."