A 15-year-old girl pleaded guilty to second-degree murder on Monday, admitting she suffocated a three-year-old toddler with his own pillow.

The girl killed Matthew Reid the day after she arrived at a foster home in Welland, Ont.

She smeared him with her blood and placed two notes under his head as he lay on the floor.

"I know what his last words were before he died," the girl wrote on a piece of paper. "Moma."

Then she signed her name.

"We haven't gotten any answers from anybody," the boy's mother, Tania, Reid said Monday outside court.

Reid said she wants to know if the girl, then 14, was a known risk for violence when she was placed in the same home as Matthew.

The girl, whose identity is protected by the Youth Criminal Justice Act, wept in the prisoner's box as facts of the case were read into court.

The Ontario Court of Justice heard she lived in various foster homes beginning at an early age and was eventually adopted, along with her biological brother and sister.

She was later removed from the adoptive home and placed in foster care in Niagara Falls, Ont., because she assaulted her younger sister.

In December 2005, the girl met a man in front of the Niagara Falls library who befriended her and started a sexual relationship.

She began visiting him in his motel room.

On Dec. 9, the teenager took her foster family's van to meet the man at the motel. As a result, she was arrested and charged with theft over $5,000.

After a bail hearing on Dec. 14, she was released from jail and placed into the care of the new foster home in Welland.

She arrived at 11 a.m. and was given a bedroom on the second floor of the house, across the hall from Matthew.

Foster mom discovers body

Assistant Crown attorney Patricia Vadacchino said Matthew was put to bed at 7:30 that night. When the foster mother checked in on the girl at 9 p.m., she noticed she was asleep with the lights off.

Vadacchino said at some point overnight, the girl lifted Matthew from his bed and placed him on the floor. She took the pillow from his bed and put it over his face. Matthew struggled as he was being suffocated, but the girl held the pillow until he stopped moving.

The girl then went back to her bedroom and stayed there until the foster mother discovered Matthew's body at 8:15 a.m., Vadacchino said.

Vadacchino quoted from notes found under Matthew's head.

"I'm the one that killed Matthew. I suffocated him. Let the cops do what they want with me," one note said. "I don't care what they do with me. Let them throw me in jail."

The Crown is seeking an adult sentence for the girl, but must first hold a suitability hearing. She will be sent to an institution in Sudbury for about six weeks to be assessed, a move agreed to by her lawyer, John Lefurgey.

The girl will return to court March 19 to confirm a sentencing date, tentatively set for April 17.