RCMP said Tuesday the boy was apprehended at the scene of the beating death of Lee Bonneau, 6.
Bonneau's body was discovered Aug. 21 in an open area behind a recreation centre on the Kahkewistahaw First Nation, a community of 600 people located about 150 kilometres east of Regina.
"Because the person investigators believe is responsible for this homicide is under the age of 12, the child cannot be charged under the Youth Criminal Justice Act," RCMP Staff Sgt. Larry Brost told news reporters Tuesday.
Brost said Bonneau died of a head trauma.
Information obtained by CBC News and confirmed by officials Tuesday shows the child believed responsible was considered a troubled boy who was known to be "violent and unpredictable."
The CBC has also learned a weapon was used. Brost said investigators were still trying to determine what the weapon was.
RCMP said no one else was involved in the death.
"There's no other person at this point in the investigation that could be responsible for this other than this child," Brost said.
When asked about why the boy attacked the other child, Brost said police did not have an immediate answer.
"This is a unique case," Brost said. "We may never find that answer."
The day of his death, Bonneau was at a community centre with his foster mother for Wednesday night bingo. She last saw him playing with a group of dogs outside, then he vanished.
After a 90-minute search, Bonneau was found with severe head injuries, and was rushed to hospital where he was pronounced dead.
He had been placed in foster care three weeks prior to his death.
The RCMP said Bonneau's biological mother described her son in a statement as "a fun-loving boy on his way to Grade 2, who had the deepest laugh and loved being outdoors."
Because the child believed responsible is under the age of 12, he cannot be found criminally responsible under Canada's Youth Criminal Justice Act.
The boy is currently in the care of social services.
On Tuesday, an official from the Ministry of Social Services said the boy was being assessed to determine what sort of treatment he needs.
"Those treatment needs are going to change as he grows older," said Andrea Brittin, an assistant deputy minister in the social services ministry. She added that the ministry and Yorkton Tribal Council child and family services agency will have to work together.
Brittin said the nature of the case means the boy is automatically deemed to be "a child in need of protection" because of an offence that would have resulted in charges under Criminal Code if he were 12 or older.
When a child in this age group is found responsible, the justice system's involvement usually ends when social services takes over, said Sanjeev Anand, the dean of the College of Law at the University of Saskatchewan and an expert on youth justice issues.
"Anyone 12 and over is caught by the, now, Youth Criminal Justice Act," said Anand. "But anyone less than 12 years of age, when they've allegedly committed an offence, has to be dealt with through some alternate means, usually through the social welfare system."
Anand said children in this situation often require a different kind of treatment, including psychological help.