A police watchdog has criticised Dyfed-Powys Police for being "inept" in its failure to investigate a rape claim.
A report by the Independent Police Complaints Commission (IPCC) found the force did not follow up the allegation in 2005 despite the suspect being a known sex offender.
The IPCC said its response was "so inept it borders on the unbelievable".
Deputy Chief Constable Jackie Roberts apologised to the unnamed woman for the "unacceptable" response.
The original claims were made when the young woman reported the rape to Dyfed-Powys Police in April 2005 alleging that she was raped 10 years earlier as a child.
The IPCC investigation found evidence to suggest a temporary detective constable, an acting detective sergeant, a detective sergeant and a detective inspector failed to ensure the rape investigation was progressed.
The claims only resurfaced during a trial for a different case centring on a rape allegation in 2009 when she appeared as a witness.
Statements made by the officers then led to the young woman facing hostile questioning and having her credibility questioned when she appeared in court.
All four of the officers have since attended misconduct meetings.
The report concluded they did not provide the quality of service a rape victim should expect of a police force and said it was a "tragedy" the case was not better investigated.
IPCC Commissioner for Wales Tom Davies said: "This woman came forward to report an alleged rape that had occurred when she was a child. This must have been a very difficult thing for her to have done and she was entitled to a proper investigation.
"The accumulation of individual and organisational failures described in our investigation led to a Dyfed-Powys Police response so inept that it borders on the unbelievable.
"Put simply, at the time that this woman reported the alleged rape to the police the man was a known sex offender on the sexual offenders' register and every effort should have been made to investigate the allegation thoroughly. It is a tragedy that it was not."
Mr Davies said the "individual and institutional failures" were made worse when two individual officers provided statements to court which led the defence in that case to question the victim about the previous allegation of rape and her version of events.
He said the force had demonstrated to the IPCC that it has since changed its systems and given assurances that the report of such a crime such could no longer be lost in the system.
He added: "It is vitally important people can come to the police with confidence that serious matters such as these are properly investigated.
"Failures of this kind though only serve to make the experience more traumatic than it already is and potentially severely dent the public's confidence.
"The force has assured me that the public can still have confidence that when they do report such serious crimes that they will be dealt with sensitively and thoroughly.
"I would expect Dyfed-Powys Police to now offer this young woman a full apology."
Deputy Chief Constable Jackie Roberts said: "This was clearly a serious allegation and our response in terms of follow-up investigation was unacceptable.
"We accept the findings of the independent IPCC investigation and the principal officers involved have been subject to misconduct proceedings.
"We are confident that our crime management procedures are now far more improved and robust than was the case in 2005 which will assist in ensuring that the situation that arose in this case should not happen again."