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One of the three bodies is removed from a home in Medicine Hat in April 2006. (CBC)

A 13-year-old Alberta girl has been found guilty of murdering her parents and younger brother in a bloody attack in their Medicine Hat home.

The jury handed down its guilty verdict on three counts of first-degree murder after just three hours of deliberations on Monday in the Court of Queen's Bench in Medicine Hat.

The girl will be sentenced Aug. 23 and faces up to six years in jail, followed by up to four years supervision in the community.

As the verdict was read Monday, the girl wept quietly in the prisoner's box, her hand placed over her mouth. Her lawyer, Tim Foster, went over to her and put his arm around her.

"You develop relationships with your client, and sometimes people just need a hug," an emotional Foster told reporters outside the courtroom.

The girl, who cannot be named under the Youth Criminal Justice Act, was accused of killing her mother, father and her eight-year-old brother on April 22, 2006, when she was just 12.

She is believed to be the youngest person in Canada to be convicted of multiple counts of first-degree murder.

During the trial, which began June 4, the girl testified that her 23-year-old boyfriend, Jeremy Steinke, broke into her home and attacked and killed her mother and father.

The girl told the court that Steinke ordered her to stab her brother, which she did once, before Steinke slit the boy's throat.

Steinke, now 24, is charged with three counts of first-degree murder, but has yet to enter a plea. He is scheduled to appear in court next week.

Judge advised jurors before deliberations

Before the jury began its deliberations, Judge Scott Brooker reminded jurors that even if Steinke physically stabbed the girl's mother, her father and brother, under Canadian law, an accused can be found guilty if they intentionally help, encourage or persuade another person to commit a crime.

The judge also told the jury that to find the accused guilty of first-degree murder, they must agree that the Crown proved there was planning and deliberation involved.

Brooker gave the jury three options on each of the three charges: find the accused guilty of first-degree murder, guilty of second-degree murder, or innocent.

After the jury's decision was announced, Crown prosecutor Stephanie Cleary said she felt satisfied.

"The Crown doesn't win and the Crown doesn't lose, but I have a duty not to prosecute a case unless I believe there is a reasonable likelihood the person will be convicted," she told reporters outside the courtroom.

"From that point of view, it's satisfying to have made what appears to have been a correct assessment of the evidence."

Girl testified she was in 'zombie' state

During the trial, the girl testified she was in a "zombie" state at the time of the killings and was unable to stop her boyfriend, or go for help.

She admitted in court that she and her boyfriend used to talk about killing her parents prior to the slayings, but she insisted she was only joking at the time.

The Crown countered that the girl was an active participant in the killings, which she plotted with her boyfriend because her parents disapproved of their relationship.

The Crown told the court the girl had plenty of opportunities during and after the killings to call 911 or go for help, but she never did.

Stabbed repeatedly

Police officers and a medical examiner told the court last month that the attack on the girl's family was bloody.

The experts testified the girl's father was stabbed 24 times, while her mother was stabbed 12 times.

The boy died from a severed jugular vein and had four stab wounds to his face and chest, the experts said, noting that an autopsy suggested he was also strangled.

The police officers and medical examiner said the family's home was drenched in blood after the attack, with splatter on the walls, floors and ceilings.

With files from the Canadian Press