West Midlands Police have admitted failing to fully investigate the Catholic church over a series of alleged child abuse attacks by a priest in the 1970s and 80s.
But they said crucial documentation that could have cleared up the controversy had been lost in the years since the abuse happened.
The admission follows a 20-year campaign for justice by an alleged victim of a former West Midland Catholic priest.
His victim, who now lives in Scotland, says West Midlands Police failed to investigate the abuse properly when it happened, and should have extradited the priest to face charges following the man's move to California.
A spokeswoman for West Midlands Police said: "We can confirm that the man has now received our findings in respect of his complaint. His complaint that West Midlands Police failed to investigate the Catholic Church has been substantiated.
"Although there was evidence of an investigation, a lack of substantiated documentation as a result of the historic nature of the case meant that we were unable to assess the thoroughness of any investigation and therefore substantiated the complaint."
The former priest, Father James Robertson, was accused of repeatedly abusing young boys in the 1970s and 80s while working at a series of churches around the West Midlands.
He was moved by the church to a parish in California as the abuse accusations raged in 1985, and was defrocked eight years later.
He has denied any wrongdoing, and says he has no intention of returning to Britain.
Despite the former priest being tracked down and confronted by one of his alleged victims in 2003, West Midlands Police said they were prevented from extraditing him by Californian state law.
A spokeswoman said: "At the time of the allegations made by the man against Father Robertson, a treaty was not in place to enable extradition from California.
"During 2006 a new treaty was ratified which means that extradition is possible. Any request for extradition is the subject of judicial procedures. West Midlands Police will review the original investigation and consider the appropriate course of action."
The victim, who is now married and in his 40s, has always maintained it is possible to arrest the man, and has even contacted Californian governor Arnold Schwarzenegger during his quest for justice.
He said he welcomed any progress on the internal investigation being carried out by the police, but was still frustrated and angry at the lack of answers presented.
"They have rather shot themselves in the foot with this; I honestly have no idea what is going on," he said. "Now I've heard about this I'm going to come back down to Birmingham to sort all of this out."
The admission by West Midlands Police came as part of an internal investigation - which is still being carried out - which was launched following complaints from the victim last year.
He maintains there should be a move to arrest the former priest, saying: "Twenty eight years have gone by during which time the West Midlands Police have failed on three occasions to investigate Father James Robertson and to bring him to justice."
Over the past 30 years, the Catholic church has been rocked by a series of allegations of abuse by priests at churches around the region.
James Robertson is alleged to have systematically abused boys while training at Oscott College, Sutton Coldfield, as a parish priest at St Elizabeth's in Foleshill, and then again at Holy Family Church in Small Heath and Our Lady of Lourdes in Cradley Heath.
He was also accused of abuse at the notorious Father Hudson's children's home in Coleshill, where paedophile priest Eric Taylor was later convicted of more than a dozen sex attacks on boys. In 2005 the archdiocese of Birmingham paid £635,684 compensation to a man who was abused as a child by Coventry priest Christopher Clonan. ..SUPL: