Justice David Watt made the ruling two days after the jury began deliberations.
Watt said the witness, a former girlfriend of one of the defendants, may have misled the court after a newspaper report that she had contradicted her sworn testimony in postings to the internet.
The 12-year-old victim, who can only be identified as “Johnathan,” was stabbed to death in November 2003.
The victim’s older brother, now 18, and two friends, ages 16 and 17, were charged with first-degree murder. All three pleaded not guilty.
The victim’s stepfather was also beaten, but managed to flee.
Because of their ages at the time of the killing, none of the defendants can be named under the terms of the Youth Criminal Justice Act.
The Crown’s key witness was a girlfriend of one of the accused. She taped a phone conversation with the three boys discussing plans to kill Johnathan and his family.
She raised an alarm within minutes, but when police arrived at Johnathan’s east end Toronto home, it was too late. They found the boy’s body, with 71 stab wounds, stuffed in a basement crawlspace.
During the trial, Johnathan’s brother acknowledged that he stabbed the victim to death, but argued that he suffered from a psychiatric disorder that made him unable to control violent impulses.
The defence claimed that the murder plan in the taped phone call was a joke and an attempt to woo the girl, who shared her then-boyfriend’s fascination with vampirism.
The witness, who was 14 at the time of the killing, testified that she considered the defendant’s blood fetish to be childish.
But a report published in the National Post said that the girl had posted comments to an internet site in which she professed an interest in “blood, pain … cemeteries [and] knives.”
It also said she had apparently posted commentary about her testimony to an online journal during the trial.
“I don’t see in this case how Humpty can be put back together again,” Watt told the court Tuesday in declaring the mistrial.
“If this young woman did not commit perjury, she came close enough … and I don’t see what else I can realistically do.” The court is scheduled to reconvene Friday.