Jeremy Steinke, seen in an undated photo, faces life in prison with no chance of parole for 25 years.

A Calgary jury has found Jeremy Steinke guilty of killing his preteen girlfriend's parents and eight-year-old brother in a premeditated plot at their home in Medicine Hat, Alta., in 2006.

The six-man, six-woman jury convicted Steinke, 25, on three counts of first-degree murder, which carry a life sentence with a minimum of 25 years in prison.

The jurors deliberated for nine hours on Thursday and another 90 minutes on Friday before reaching their decision.

Steinke showed no emotion, keeping the same posture he has throughout the three-week trial with his head bowed and his eyes fixed on the floor.

His mother, Jacqueline May, wept quietly as the jurors said "I agree" one after the other.

Court of Queen's Bench Judge Adele Kent instructed jurors Thursday that they must believe the slayings were planned and deliberate in order to convict Steinke of first-degree murder.

Jacqueline May, Jeremy Steinke's mother, had no comment for reporters outside the courthouse Friday. She wears an oxygen tank because of a lung disease. (CBC)

Crown prosecutor Ramona Robins asserted that Steinke and the girl, then 12, plotted the killings in order to run away together. Robins portrayed Steinke as a lovestruck, immature man who would do anything to keep the preteen girl's affection.

A string of Steinke's friends testified that he complained angrily about the parents' objections, and asked for help to get rid of them because he was worried the girl would leave him if he didn't.

In an audio recording of a conversation between Steinke and an undercover officer played at the trial, he joked and bragged about the murders, laying out the bloody events of the night.

Steinke had testified that he fatally stabbed the girl's parents in their southern Alberta home in April 2006, but that it was impulsive and not planned. Wearing a neoprene mask and carrying a knife, he was only waiting for the girl in the home's dark basement when her mother surprised him, Steinke told the jury.

He also repeatedly denied killing the girl's eight-year-old brother, saying that she slashed the boy's throat while he watched from a doorway.

The victim family's bloody bodies were found in their southern Alberta home in April 2006. (CBC)

The Crown said that forensic evidence, including blood found on Steinke's clothes, proved the accused was present for all three slayings, and that e-mails between him and the girl before the attacks show premeditation.

The girl's father was stabbed 24 times, including nine times in the back, and suffered several defensive wounds, while the mother was stabbed 12 times, including a 12-centimetre-deep wound that pierced her heart, the medical examiner in the case concluded.

The boy suffered five stab wounds, including a wide, deep slash across his neck. He was found lying on his bed, surrounded by blood-splattered toys.

Crime scene photos, particularly of the boy, brought some jurors to tears during the trial.

"I think what the public can take away from this incident is that there's heroes in this story, and not just villains in this story," said Robins after the verdict.

"A father who fought for his family's life to his death. You had [Medicine Hat] police officers who mourned the loss 2½ years later of the terrible things they saw. The police officers from Leader [Saskatchewan] who on a slight lead that maybe these people would come through actually caught them by doing their own stakeout."

Defence argued for lesser charges

The jury heard that Steinke grew up being abused by his alcoholic mother's ex-husbands and bullied by classmates. At 13, Steinke was diagnosed with depression and hyperactivity, and he later tried to hang himself, his mother told the court.

Defence lawyer Alain Hepner had argued that Steinke was in an alcohol- and drug-fuelled haze when he "snapped" and stabbed the girl's parents.

He tried to convince the jury that lesser charges of second-degree murder, or even manslaughter, were more appropriate convictions in the parents' deaths. Hepner also argued for an acquittal for Steinke in the killing of the boy.

The victims cannot be named to protect the identity of the girl, who is now 15 and serving a maximum 10-year youth sentence in an Edmonton psychiatric hospital. She was convicted of three counts of first-degree murder last year.

Crown may examine underage relationship

Robins said her office could consider additional charges against Steinke on the matter of his relationship with the much younger girl, but that the priority was to complete the murder trial.

"There's been a lot of statements — they're boyfriend and girlfriend — and that's not accurate. She was 12 years old and not capable of consent," said the prosecutor.

'A lot of people were personally affected by this tragedy and I thought it would be important for them to see the end of it.'—Ramona Robins, Crown prosecutor

Steinke's trial was moved to Calgary from Medicine Hat because a judge agreed with Hepner that there would be a larger pool of unbiased jurors in a case that had already created headlines around the world.

Steinke will be sentenced Dec. 15 in Medicine Hat, when relatives of the slain family, who plan to travel from Ontario, will give victim impact statements.

The Crown asked that the sentencing be moved back to the city, about 300 kilometres southeast of Calgary.

"I felt it was really important for the community to see closure," explained Robins on Friday. "We are a small community. It's only 60,000 people, and a lot of people were personally affected by this tragedy and I thought it would be important for them to see the end of it."

Victim impact statements do not sway a minimum sentence, but become part of the record for a parole board to review in future decisions, explained Robins.

With files from Bryan Labby and the Canadian Press