Police have been urged to focus on catching the gang that carried out the Loughinisland massacre after it emerged they were probing the alleged theft of a case file.
Detectives are investigating the apparent leaking of "sensitive documents" from the Police Ombudsman's Office, which are understood to have been used in a film about the 1994 atrocity.
It comes days after a loyalist, Ronald Hawthorne, was named for the first time by the No Stone Unturned documentary and by the media as a suspected gunman in the attack.
The film was shown to relatives of those killed in the massacre last night. It will go on general release shortly.
Mr Hawthorne was arrested following the attack but was never charged.
Six men died in the Heights Bar in the village on June 18, 1994 as they watched the Republic of Ireland play Italy in a World Cup match.
The UVF claimed responsibility, but no one has ever been convicted of the murders.
Former South Down MP Margaret Ritchie said the PSNI investigation must not be distracted by the leaking of the file.
"I have a very clear view that those who carried out the execution of six men have to be apprehended," she said.
"We need prosecutions and convictions. The families deserve justice. I would hope that the police are not trying to distract from the main focus, which is the need to get prosecutions and convictions in relation to this massacre.
"The focus has to be on what is in the documentary and most importantly pursuing those who undertook this terrible crime and there should be no distractions. Where is the PSNI inquiry and what arrests have they made and what investigation has taken place since the Police Ombudsman report was published in June of last year?"
The victims of Loughinisland were Adrian Rogan (34); Patrick O'Hare (35); Eamon Byrne (39); Malcolm Jenkinson (53); Daniel McCreanor (59), and Barney Green (87).
All six were Catholics. Mr Green was one of the oldest people to be murdered during the Troubles.
Five other people shot by the gunmen survived.
No Stone Unturned, by Oscar-winning American documentary maker Alex Gibney, examines collusion around the massacre.
It alleges that Hawthorne's wife named her husband as one of the killers in two phone calls to police. It further claims that Hawthorne is the man referred to as 'Person A' in a Police Ombudsman's report from last year.
In a statement, PSNI Assistant Chief Constable Stephen Martin said it was carrying out an impact assessment of the suspected theft of sensitive material apparently used in the making of the documentary.
The head of the PSNI's Crime Operations Department said that detectives would also be examining the film.
ACC Martin stated: "We are aware that sensitive documents relating to the Loughinisland investigation, which we believe originated from within the Office of the Police Ombudsman of Northern Ireland (PONI), are suspected of being used in the No Stone Unturned film.
"We are carrying out an assessment of any impact which may be caused as a result of the suspected theft of this sensitive material.
"Durham Constabulary have been conducting an independent criminal investigation for a number of months into how a solicitor's firm came into possession of documents containing sensitive information from PONI. While at this stage there is nothing to suggest these matters are linked, the PSNI has requested the existing investigation is extended to include this recent suspected theft of sensitive material.
"These were appalling murders carried out by truly evil people and I am very aware of the hurt and anger felt by the families of those killed and those injured. The PSNI remains firmly committed to apprehending those responsible for these murders and detectives will be reviewing the No Stone Unturned film.
"I would also like to take this opportunity to appeal to members of the community to come forward if they have any information which may assist us.
"At this stage the PSNI is confident to continue sharing sensitive information with PONI following the completion of an independent review in July in respect of PONI information handling processes.
"This provided collective reassurance that any information shared with PONI is handled and stored to the expected high standards in line with the Government Security Policy Framework and our shared Memorandum of Understanding."
An Ombudsman Office spokesman said: "We welcome the PSNI's continued commitment to catching the terrorists responsible and fully support their appeal for anyone with information about the murders to come forward. This office is continuing to examine concerns which arose during our Loughinisland investigation that certain RUC officers had relationships with and provided information to loyalist paramilitaries.
"We will fully co-operate with the police investigation into what the film has reported as a document received by a journalist in 2011."