Agencies involved in the care of three teenagers who all died in separate cases have apologised.
Swansea council promised to "learn lessons" after reports showed failings in the way they were looked after.
The three, two girls and one boy, were all 16 when they died: two were drug-related and one was a suicide.
One was Carly Townsend, whose mother and half-sister were jailed for manslaughter after they put her to bed following her heroin overdose.
The deaths, which happened between 2007 and 2008, were unconnected, but the findings of three separate serious case reviews have now been published.
The reviews, commissioned by Swansea Safeguarding Children Board, found that not enough was done to protect them and agencies involved were found to have failed to properly communicate with one other.
The care of the children involved several agencies including the council, police, schools and doctors.
Swansea council said it fully accepted the reviews' recommendations and action had been taken to improve services.
The names of the teenagers in the serious case reviews have not been released, but BBC Wales understands one of them, identified only as Child B, is Carly Townsend, who died in May 2007.
The serious case review identified that at several key points, information sharing between key agencies involved with her in the report, was "inadequate".
The report said: "The review found that the decisions made and actions taken in Child B's case were not based on careful assessments.
"Actions and decisions were driven by events or by what Child B and her family determined would happen."
Carly had been staying at a secure unit in Neath when she was released back to the family home.
The report said: "It is now known that on the day of her release Child B took heroin. The next day she breached her curfew and her mother gave a false explanation to protect her."
A review meeting organised by the youth offending team and local authority was held a week after her release and she appeared "well".
Her mother, Andrea Townsend, and her half-sister, Gemma Evans, put her to bed despite her suffering ill-effects from the drug, and then watched television rather than calling an ambulance.
Both were both convicted of manslaughter through gross negligence.
The report also criticised the Probation Service over its handling of Carly's case.
"Over the whole period of the scope of the review, the Probation Service had considerable involvement with Child B's mother and sister, who both had chronic involvement in criminal activity and drug dependence," it said.
"The probation report comments that Child B's needs were not adequately considered and there was a lack of effective engagement between probation and other agencies."
The two other reports also concern teenagers who had come into contact with care agencies.
Child D, who was almost 17, died in his bedroom at home in Swansea.
The report said he had attended his last day of school in Newport in 2002 when he was 11. Following a move to Swansea, he was not registered to attend any school.
Child E, who took her life in 2008, was the subject of "areas of concern" including ongoing neglect, self harm, substance misuse, problems at school and unresolved emotional issues.
She and her family had been known to, and received services from, a number of agencies throughout her life.
Chris Maggs, chair of Swansea Safeguarding Children Board, said: "On behalf of the agencies I want to say we are very sorry for letting these children down and express our condolences to their families.
"We are determined to learn the lessons of these reviews. We are implementing all of the recommendations and monitoring their effectiveness.
"One of the major lessons is that agencies should provide training and coordination to better engage those vulnerable children and families who may be unwilling to accept our support.
"The safety of children is the most important matter for the board and I want to reassure people that we have already made a number of changes and improvements."
Steve Walker, Swansea council's head of child and family services, said: "We fully accept the recommendations in the serious case reviews and I would like to reassure people that over the last two years we have taken a range of actions to help minimise the risk of this happening in the future."
He said there was now better information sharing with partners so that all the agencies "have a more complete picture of the relevant issues before deciding collectively how best to protect a vulnerable child".
A team of experts was appointed last year to oversee the work of Swansea council's social services department.