Six-year-old Ethan Flett (top) and Gerrod Head, 7, died after they became trapped in a burning shed on the Opaskwayak Cree Nation in northern Manitoba. (Family photos)The fire that killed two boys on the Opaskwayak Cree Nation this week was started by a group of kids playing inside a locked shed, police said on Wednesday.

Ethan Flett, 6, and Gerrod Head, 7, died after they became trapped in a wooden garden shed in the yard of a house that had been destroyed by a suspicious fire earlier this year.

Investigators believe the two boys, who were cousins, were among a group of six young children playing in the yard after school on Tuesday. The children crawled through a small hole at the back of the locked shed, which had been storing tools and other items.

The children were playing with fire, RCMP said Wednesday, and were unable to find their way out of the shed when the blaze got out of control. The family has been told the two children were overcome by smoke.

'They were inseparable'

The boys' grandmother, Mary Head, said she had thought they were playing in their rooms after they got home from school on Tuesday.

"They usually tell me where they're going to go. I didn't even hear them going out," she said.

Children reflect on the scene of the fatal fire. (CBC)"Then I talked to my daughter and I said, 'Where's the boys?' and she said, 'They're not here,' and she came running outside and they weren't here and we were calling them. The next thing, we see … a big smoke over here."

The family and others on the northern Manitoba reserve immediately went looking for the boys, but didn't find them in time.

"They were always together. They were inseparable," she said.

"They told us they didn't suffer long."

The fire commissioner is still investigating the blaze, but RCMP say foul play is not suspected.