TORONTO - Ontario's highest court has ruled the family of a teen fatally shot by Toronto police can't sue the province's police watchdog over its investigation of the incident.

Duane Christian, 15, was shot by police on June 20, 2006, while driving a stolen minivan in the parking lot of an east-end highrise apartment building.

The Special Investigations Unit cleared the officers of wrongdoing, but Christian's family alleged the investigation was incompetent and filed a $2-million negligence lawsuit.

The officer who shot Christian, Const. Steve Darnley, told an inquest last year that he had no other choice but to kill the teen, who was trying to run over his partner.

Christian's family also argued that allowing the families of victims of police shootings to sue the SIU for negligent investigation would lead to improved investigations.

The Ontario Court of Appeal dismissed the suit, ruling that SIU's duty in its investigations is to the public at large and not to victims or their families.

"Recognizing a duty of care in favour of victims and their families could interfere with the SIU heeding its primary duty to the public at large," Justice Robert Sharpe wrote in the unanimous decision released Friday.

The SIU ought to be free to make decisions without being subjected to a private law duty of care to specific members of the general public, Sharpe noted.

Imposing a private law duty of care would "introduce an element seriously at odds with the fundamental role of the SIU to investigate allegations of criminal misconduct in the public interest," Sharpe said.

The inquest jury recommended that the province consider enacting a policy that would prohibit police officers from using their weapons in circumstances similar to the Christian case.

The police union has said it's opposed to such a policy, saying it doesn't give officers the opportunity to protect themselves in all circumstances.