An Alberta boy who was allegedly murdered by his 12-year-old sister was stabbed at least once outside the bedroom, where he was later found with his throat slashed, a police blood spatter expert testified Tuesday.

RCMP Sgt. Serge Larocque said police found more than 100 drops of the eight-year-old boy's blood all along the carpeted hallway leading to his room in his family's home in Medicine Hat, southeast of Calgary.

There was also blood in the stairwell leading to the basement wherethe boy'sparents were found slain, Larocque said.

"This is consistent with a blood-letting event occurring in this area," Larocque told the jury at the first-degree murder trial of the boy's sister, who is now 13.

The boy's diluted blood was also on a self-sharpening kitchen knife that had been rinsed off in the upstairs bathroom, he said.

Police officers and a medical examiner have already testified that there was blood everywhere in the house and the victims all died from massive blood loss in April 2006.

On Monday, the jury heard that the boy died from a severed jugular vein and had four stab wounds to his face and chest. An autopsy revealed pinpoint bleeding on the inside of his eyelids and mouth — which suggests strangulation.

Chaotic scene in the basement

Larocque, who works for the RCMP major crimes section in Edmonton, described how he was called to assist Medicine Hat police hours after they discovered a "major crime scene."

He spent 11 hours the next day trying to piece together what had happened from the hundreds of blood drops, spray and smears throughout the house.

In clinical detail, Larocque painted a picture of a chaotic scene in the basement.

The mother, who had been stabbed 12 times, including inheraorta and lungs, probably died at the bottom of the basement stairs where she'd been attacked, he said. Her blood was splattered on the floor, walls and ceiling.

The father, who suffered 24 stab wounds and countless slashes, appeared to have been involved in a mighty struggle that left his blood splattered from one end of the basement to the other. The area around a shattered staircase banister was also covered with blood.

"I imagine there was a fair amount of movement in the south end of the family room," Larocque said.

Father's blood on futon, fireplace, ceiling

Police forensics specialists found samples of the father's blood on nearly every object in the basement, including exercise equipment, a futon bed, the ceiling and on a gas fireplace.

During cross-examination, Larocque said the evidence allowed him to conclude the parents died in the basement and didn't come upstairs, because there would have been more of a bloody trail.

Larocque used a slide show of crime scene photos to accompany his testimony — the first time that pictures of the blood-soaked house and victims were shown in the courtroom.

Several people in the public gallery dabbed their eyes and bowed their heads as graphic photos of the three deceased were shown on a television screen.

The girl can't be identified under provisions of the Youth Criminal Justice Act.

Her co-accused and boyfriend at the time of the crimes, 24-year-old Jeremy Steinke, will face the same three charges of first-degree murder at a later date.

The two were arrested two hours east of Medicine Hat in Leader, Sask., a day after the bodies were discovered.