Police are warning they "cannot rule out" the possibility bodies could be found in the rubble of a Sydney building destroyed by fire.
Cadaver-detection dogs will be used to assist investigators sorting through the debris, as police begin preparing the heritage building for demolition.
A fourth young boy, believed to be at the scene when the blaze broke out last Thursday, will be questioned by police.
Three other children, a 12-year-old and two 13-year-old boys, are already assisting detectives with their investigation.
Detective Superintendent Gordon Arbinja said "it's possible" charges would be laid against them.
"We'll go through the whole investigation and then we'll work out what the best and most appropriate measure is to solve this matter," he said.
He would not reveal what the teenagers have told police.
"It would be harrowing for them, but their parents are with them, or a support person," he said.
He expects to speak to more teenagers as investigations continue.
Unstable walls continue to pose a threat to public safety, and to the stability of surrounding buildings.
A fire is still burning deep under the rubble.
Two homeless people believed to have been sleeping at the building remain unaccounted for.
"We can't rule out that there were people under the rubble. We can't rule it out," Detective Superintendent Arbinja said.
"I don't have their identities.
"I just have a description of them, but we need to do the best we can do to account for these people."
More than 100 people were displaced on Thursday, when an out-of-control fire tore through the former heritage-listed hat factory in Surry Hills.
Fire and Rescue NSW has begun preparing the site for demolition.
"That includes clearing the large rubble, large piles of bricks," Superintendent Adam Dewberry said.
"It is a major operation, it's a major job to get this building down especially in the compressed time frame we're talking about," he said.
Part of the process includes restoring electricity and gas to houses and businesses in the area.
Clean-up work to ensure the building is safe is expected to take several days.
Heritage architect Hector Abrahams said "quite a bit of the walls" of the burnt building could be saved.
He said bricks manufactured in the early 20th century can survive fires quite well, and the thick walls were likely to be "structurally viable".
"If you were going to have any building survive this experience it would be a brick warehouse of this period," he said.
"It's as good a candidate you're going to get."
