Details of a convicted killer's past offences could have been introduced in the Arlene Arkinson murder trial, the Public Prosecution Service has said.
Arlene Arkinson went missing after attending a disco in Donegal
It decided not to put Robert Howard's convictions before the judge because of legal advice, the service said.
It comes as the Police Ombudsman confirmed it was investigating the handling of the police inquiry into 15-year-old Arlene Arkinson's murder.
Howard was jailed for life for killing London schoolgirl Hannah Williams.
The convicted child abuser and rapist was found not guilty in June of killing Arlene in 1994.
The Public Prosecution Service said in a statement that senior counsel had told them there was not a reasonable prospect of the court agreeing to introduce evidence of Howard's past offences and sexual misconduct.
After Howard's acquittal, news reports had claimed that legislation allowing evidence of bad character to be introduced came into effect too late for the murder trial.
However, it has emerged since that the new law was in force before the jury was sworn in.
Meanwhile, the police's handling of the Arlene Arkinson murder inquiry is being investigated by the Police Ombudsman.
They are also liaising with the chief constable to investigate how a retired senior officer still has copies of official police files on the killing.
During an interview for UTV's Insight, Eric Anderson produced what he said was official paperwork.
Arlene's family are planning to speak with Police Ombudsman's officers.
The office confirmed it had received a complaint and was actively examining the police investigation into the murder of the schoolgirl.
Mr Anderson, a former chief superintendent who also investigated the Omagh bombing, commented about Arlene's murder after Howard was cleared of her killing.
The retired detective is now also being investigated by the Ombudsman after he told a UTV reporter that since his retirement he had held on to police files relating to the case.
A spokeswoman for the PSNI said it was their current policy that all official documents, forms and other records generated in the course of police work remain the property of the chief constable.
Sinn Fein MP Pat Doherty said: "People will now watch closely the response of the PSNI to all of this."
The SDLP's Alban Maginness said: "It is clear that a full inquiry must be conducted immediately into this matter."