Anger over parental disapproval about her much-older boyfriend led a Medicine Hat girl to murder her family, a trial heard.

Crown prosecutor Stephanie Cleary read the following to the jury Monday during the opening of the first-degree murder trial:

"'I hate them. So I have a plan. It begins with me killing them, and ends with me living with you,'" the girl is alleged to have said to some friends in a posting on a website.

The girl, now 13, was 12 at the time of the April 2006 killings. Her boyfriend was 23 years old.

The Crown intends to present a drawing, taken from the girl's school locker, which seems to depict the killings and an escape in the boyfriend's truck.

Police arrested the couple in Leader, Saskatchewan. Bloody clothes had been found in the truck.

"What she says isn't evidence. It's just what she expects the evidence will turn out to be," defence lawyer Tim Foster told reporters afterwards about Cleary's opening remarks.

The jury heard some evidence. Police testified that they found two knives. The girl's 42-year-old father, found in his underwear, had suffered 24 stab wounds. The 48-year-old mother, naked from the waist down, had been stabbed 12 times.

There is evidence to show both had struggled with their attackers.

The girl's eight-year-old brother had been strangled and stabbed. A cut to the neck was likely the fatal blow.

One police officer choked up during testimony. The accused appeared upset too.

Foster said of his client: "This is obviously very difficult for her. Regardless of what the jury may ultimately decide, this is a very tragic case."

Cleary agreed this is a difficult case for all those involved.

Seven men and five women comprise the trial's jury. The case is expected to last about five weeks.

Jeremy Allan Steinke, 24, the girl's boyfriend at the time of the slayings, will have a trial date set next month.

Kacy Lancaster, 19, and a 16-year-old girl, have been charged with accessory to murder for allegedly trying to help cover up the crime.

With reports from CTV's Sarah Galashan and Shelly Makrugin, plus files from The Canadian Press