Sydney - A teenage boy shown hooded and shackled in images from a juvenile detention centre that shocked Australia has apologised for his crimes as anger mounted over the scandal on Wednesday.The treatment of Dylan Voller has become the focus of outrage after graphic evidence was broadcast of prison guards assaulting mostly indigenous boys in the Northern Territory, including stripping them naked and using tear gas.It prompted an appalled Prime Minister Malcolm Turnbull to order an inquiry into the treatment of young inmates.In one video from last year, Voller, then 17, is shown hooded and shackled to a mechanical restraint chair and left alone for two hours.A former guard at the Don Dale Centre in Darwin told the Australian Broadcasting Corporation, whose Four Corners programme exposed the abuse on Monday, that he had seen the boy put in the chair at least three times.Voller, who has been in and out of custody since he was 11 and who is now in an adult prison, thanked the ABC for "getting the truth out there" in a hand-written letter released by his lawyer on Wednesday."I would just like to thank the whole Australian community for the support you have shown for us boys as well as our families," said Voller, who has previously been convicted of crimes including car theft and robbery."I would also like to take the opportunity to apologise to the community for my wrongs and I can't wait to get out and make up for them."Northern Territory Chief Minister Adam Giles has insisted he was not aware of the extent of the abuse, as calls mounted for him to resign after comments he made in 2010 urging criminals to be put "in a big concrete hole" resurfaced on Wednesday.The Northern Territory News headlined its front-page "Sack the lot of them", while The Australian broadsheet said in an editorial: "Juvenile detention horror a window on our shame."Rights groups have called for the royal commission to be Australia-wide, but Turnbull ruled this out, saying it risked becoming too cumbersome."This royal commission, which will be done in collaboration with the Northern Territory government, will be focused on the youth detention centres and youth detention practices in the Northern Territory that were the subject of the Four Corners program," he said on Wednesday.The Northern Territory has one of the highest crime rates in Australia, with indigenous offenders making up more than two-thirds of the prison population.According to Amnesty International, Aboriginal children are 26 times more likely to be jailed than their non-indigenous counterparts as they struggle to deal with poor education, high unemployment rates, and substance abuse.