WINNIPEG -- As five-year-old Phoenix Sinclair suffered a final, deadly beating on the concrete basement floor of her family's home, a terrified stepbrother peeked around the corner and witnessed the assault, an inquiry into the girl's death heard Monday.

"I was just a little kid, you know, didn't even know what ... was going on," testified the stepbrother, who is now a 20-year-old man and cannot be identified under a publication ban.

Phoenix's mother, Samantha Kematch, was sitting on the stairs and watched her new boyfriend, Karl McKay, punch and kick Phoenix for 15 to 20 minutes on that day in June 2005, the stepbrother said. Afterward, the couple left and he, then a boy of 12, went to check on the battered girl.

"She was all cold. I put my hand by her mouth. She wasn't even breathing."

Later, McKay and Kematch came back and tried to revive the girl.

"They brought her upstairs, put her in a bathtub, tried running water on her, tried to do CPR."

The inquiry is examining how Manitoba child welfare failed to protect Phoenix, who had spent much of her life in foster care or with family friends before being returned to Kematch. Months before Phoenix's death, social workers paid a short visit to Kematch, didn't actually see the child, but decided all was well.

The beating that killed Phoenix was not out of the ordinary. The stepbrother and another sibling witnessed Phoenix being abused, neglected, starved, shot with a BB gun and forced to eat her own vomit. Other children in the home were not physically abused.

Kematch and McKay buried Phoenix in a shallow grave near a landfill and told others she was still alive. They continued to collect welfare benefits with her listed as a dependent.

Eight months later, one of the child's stepbrothers told his mother -- a former common-law spouse of McKay -- and she called authorities. Kematch and McKay were arrested a short time later and were convicted in 2008 of first-degree murder.

McKay's former partner, whose identity is also protected under a publication ban, told the inquiry Monday that McKay had always been a violent man and had tried to kill her twice in the five years they were together.

"He always fought me where the bruises wouldn't show. It was always under my clothes."

On one occasion, she testified, McKay tried to throw her and their infant son down a staircase. Another time, he came at her with a machete.

"(McKay's) sister gave me a machete because I was alone all the time, and he tried to use that machete on me."

McKay also had a long record of domestic violence outlined in the province's family services central database. But social workers never caught on that he had become part of Phoenix's life.

The inquiry has already heard of a list of failures by social workers.

Social workers were sometimes unaware of who was taking care of Phoenix -- usually it was friends of the family or relatives, for days or weeks at a time. In 2003, she was seized from her biological father's home after a day-long drinking party where suspected gang members were present.

The father was told to undergo alcohol counselling before he could get his daughter back. He didn't, but regained custody anyway.

The step-brother's mother who testified Monday lashed out at social workers at one point for their handling of Phoenix's case.

"Social workers failed her."