Vulnerable teenagers in care are being placed at risk of abuse while living in unregulated homes in England and Wales, a BBC News investigation has found.
At least 14 council investigations have been launched into "organised and complex abuse" in the past four years.
BBC News also obtained a confidential briefing reporting "significant failings" and spoke to one girl who had been trafficked and abused.
The government said children in care or leaving care "deserve to be kept safe".
Children over the age of 16 are increasingly being placed in unregulated homes, often known as semi-independent or supported accommodation.
As they are deemed to be providing support rather than care, they are not inspected by a regulator in England and Wales, despite the vulnerabilities of many of the children.
Freedom of Information requests to all UK councils revealed 13 investigations involving unregulated homes in England and one in Wales launched in the past four years.
The NSPCC says this suggests "young people who need support are being exposed to serious dangers".
"Organised and complex abuse" is defined as "abuse involving one or more abusers and a number of related or non-related abused children" by the London Child Safeguarding Board.
Incidents do not necessarily involve the staff themselves.
One of the investigations concerned children and young people living in homes in Essex and London run by a company called Centurion Care.
BBC News obtained a confidential briefing sent around councils, claiming there were "significant and numerous safeguarding failings".
Many of the children who lived in the homes - closed in 2017 - had faced some of the most challenging home lives imaginable and some had been involved in crime from an early age.
Carla spent years in foster homes before being sent to Centurion Care.
She had a history of self-harm, which continued inside one of the company's homes in Basildon.
"There was a situation where it'd been really bad, I'd lost a lot of blood," she says.
But she says when she asked a member of staff to be taken to hospital, he said he could not leave the other residents alone - and there were no bandages for her to use.
"He was like, 'Oh, you should go walk to the shop,'" she says.
She says an hour later, she was taken to a pharmacy.
And on another occasion, she took an overdose but says she was not taken to hospital until the next day.
"The staff shrugged it off," she says.
Carla says she overdosed on three occasions while living in the home - but Centurion Care told BBC News it was aware of only one incident and the NHS 111 non-emergency telephone service had advised she did not require hospital care.
All its homes had first-aid kits, it added.
Andy worked as a support worker across many of Centurion Care's homes, his first job working with young people.
"They were all very high risk - sexually exploited kids, drugs and alcohol abuse, some that had disabilities - all [under] one roof," he says.
The homes were "completely wild", with residents keeping drugs and large amounts of cash in their rooms, and Andy says he felt powerless to intervene.
"There was nothing you could have really done about it because the other staff members didn't do anything about it," he says.
The confidential briefing says one of the homes was under surveillance by Essex Police "over concerns around drug dealing and criminal gang activity", while other young people lived inside.
Centurion Care said it had been aware of the police surveillance, had worked with the authorities and had introduced a CCTV system across all its homes to prevent drug dealing.
Essex Police declined to say when the police surveillance had ended and how long residents had remained in the property.
Andy says one child's story troubled him more than any other - a girl that frequently went missing.
And he remembers looking out the window when he saw her for the last time.
"I just saw a bunch of boys in a car and she just jumped in," he says.
'No-one deserves that'
She was missing for more than a week before being found in the West Midlands, where she had been abused.
"It was the worst, no-one deserves that," she says. "Whoever did what they did, someone needs to pay a price for the pain."
Like a large number of children in care, she had been placed in a home outside of her local authority. She says she did not know the area at all and did not have any friends.
"I was always just running away, trying to get away from the home," she says.
There is no suggestion Centurion Care staff were involved in her trafficking.
It said they had all had local-authority safeguarding training and it had had relevant policies and procedures in place, including notifying the authorities.
One council that placed children in Centurion Care homes said it was unable to comment on "police matters" or "individual cases". Another said it had removed children as soon as it had become aware of concerns.
Local authorities are responsible for checks on unregulated homes in England and Wales. Many conduct unannounced visits but there is no mandatory inspection regime. In Scotland and Northern Ireland, the homes are regulated although not to the same the standard as children's homes.
The Department for Education in England declined to be interviewed.
In a statement, it said: "Children in care or those leaving care, including older children, deserve to be kept safe in good quality accommodation.
"Councils have a legal duty to make sure accommodation for these children is suitable.
"We have written to all directors of children's services to remind them of this duty and we are working with the sector and with Ofsted to bear down on issues related to poor practice in the use of semi-independent accommodation."
What are your experiences of unregulated homes or semi-independent accommodation? Email email@example.com.
Please include a contact number if you are willing to speak to a BBC journalist. You can also contact us in the following ways: