A collective gasp erupted in a Toronto courtroom Friday as a teen girl was found guilty of first-degree murder in the death of Stefanie Rengel.

Prosecutors immediately said they would ask for an adult sentence for the 17-year-old girl, known only as M.T.

She broke into tears as the verdict was read.

Rengel's family members sobbed quietly and hugged each other on one side of the courtroom while the relatives of M.T. wept on the other side.

As a young offender, M.T. faces a maximum sentence of 10 years — six in jail and four under supervision in the community. An adult, first-degree murder conviction carries a life sentence with no chance of parole for 25 years. She is scheduled to be sentenced next Tuesday.

Crown prosecutor Robin Flumerfelt read a statement on behalf of the Rengel family outside court:

Rengel family relieved

"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 families as well.

"We're most grateful to the jurors who had their own lives put on hold and were tasked with the responsibility none of us would want regardless of the verdict.

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

Defence lawyer Marshall Sack called the tragedy of Rengel's murder and the conviction of his client "absolutely overwhelming."

Toronto police Det.-Sgt. Steve Ryan, who investigated the case, said he was both relieved and saddened by the verdict because it destroyed the lives of two families.

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

Murder discussed via internet

Rengel, 14, was found bleeding from six stab wounds in a snowbank outside her parents' home on New Year's Day 2008.

Prosecutors argued that M.T., driven by jealousy, used sexual blackmail via the internet to pressure her boyfriend, now 19, to kill Rengel.

His first-degree murder trial is scheduled for this fall.

The defence had argued the jury shouldn't draw murderous intent from messages sent over the Internet.

Jury wades through 30,000 pages

The jury of three women and nine men reached their verdict after three days of deliberations.

They considered 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 girl appeared to suggest to her boyfriend she wanted Rengel dead.

The boyfriend, who once dated Rengel, writes: "What about Stef," according to transcripts filed with the court.

"Bang, bang," she replies.

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

She writes: "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."

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

The pair, who were both under 18 at the time, cannot be identified under the Youth Criminal Justice Act.

Corrections and Clarifications

  • The jury was not made up of nine women and three men, as originally reported. In fact, there were three female jurors and nine men. (March 20, 2009 | 9:15 p.m. ET)
With files from the Canadian Press