A Manitoba teen has pleaded guilty to first-degree murder in the 2015 killing of 11-year-old Teresa Robinson.

The girl's remains were discovered May 11, 2015, six days after she was last seen leaving a children's birthday party on the Garden Hill First Nation, a fly-in community about 500 kilometres northeast of Winnipeg.

Initially, investigators believed Teresa was killed by a wild animal but later determined it was a homicide. The upper half of Robinson's body was missing, having been eaten by animals, and her skull was found 30 feet from her remains, court heard.

An autopsy revealed she had been sexually assaulted. Killings committed during the course of a sexual assault are automatically assessed to be first-degree murder.

During what Crown attorney Chris Vanderhooft described as a "lengthy and protracted investigation," hundreds of local residents provided police with voluntary DNA samples to aid in the search for Robinson's killer.

"It was in the course of that sweep that RCMP took a consent DNA sample from the accused," Vanderhooft said.

Police arrested the boy, who is now 17, in March 2016.

"He is prepared to admit [the murder] happened in the course of a sexual assault," his lawyer, Katherine Dowle, told court.

The accused will return to court for sentencing Feb. 5, at which time Crown and defence lawyers will jointly recommend he receive the maximum youth sentence of 10 years custody and community supervision.

Vanderhooft said a more detailed account of the killing and its circumstances will be included in an agreed statement of facts to be provided to court prior to sentencing.

The boy can't be named under provisions of the Youth Criminal Justice Act.

Outside court, Teresa's mother said the boy's guilty plea would bring her some closure.

"All he kept saying was that he didn't … do that, he wouldn't do that," Sandra Robinson said.

She said she hasn't yet turned her mind to what she wants to tell her daughter's killer when given a chance.

"I haven't even thought about that," she said.

Teresa was well-liked in her community, Robinson said. "She used to make people laugh. She was outgoing, fun to be around. Everybody liked her. Everybody loved her.

"She was a little girl. She was my baby."