The producer of the No Stone Unturned documentary and a journalist involved in the film have been arrested in connection with the suspected theft of confidential documents held by the Police Ombudsman of Northern Ireland relating to the Loughinisland massacre.
The documents relate to a police investigation into the murder of six men at Loughinisland in 1994.
The film concerned is No Stone Unturned, which drew considerable controversy upon its release in November last year for naming one of the suspected gunmen.
Alex Gibney, who made the film, confirmed on social media that another producer of the film Trevor Birney and journalist Barry McCaffrey were the two men arrested.
He called the arrests "outrageous".
Alert: @trevorbirney my producer on "No Stone Unturned," and journalist @Barry_TheDetail have just been arrested in Belfast, NI, for doing good, hard-hitting journalism. Outrageous. Raise ruckus. I am traveling. More later. https://t.co/gOAunwqrxA— Alex Gibney (@alexgibneyfilm) August 31, 2018
Solicitors representing both Mr Birney and Mr McCaffrey have also confirmed their detention to the Press Association.
A spokesperson for Fine Point Films, who made the documentary, told the BBC: "We can confirm that officers from the PSNI and Durham Constabulary are currently speaking to our team and until that process is concluded we cannot make any further comment."
Both journalists were key players in uncovering material regarding the murders in 1994 and allegations of collusion linked to the killings.
Officials from the Ombudsman office reported the theft to PSNI, who in turn asked Durham Constabulary to conduct an independent investigation.
Detectives from Durham Constabulary, supported by officers from the PSNI, executed search warrants at three properties in the Belfast area on Friday.
Two residential properties and a business premises were searched.
Police said two men, aged 51 and 48, were arrested on suspicion of theft during this operation on Friday morning and are currently being questioned at Musgrave Police Station.
A number of documents and computer equipment seized during the raids will be examined by specialist officers over the coming days.
The inquiry centres on the theft of sensitive material, which was used in a documentary film re-examining the 1994 murders.
A spokesman for Durham Constabulary said: “This morning’s arrests are a significant development in what has been a complex investigation.
“The terms of reference given to our inquiry were clear in that the investigation is solely into the alleged theft of material from PONI.
“The theft of these documents potentially puts lives at risk and we will follow the evidence wherever it leads us.”
A spokeswoman for the Police Ombudsman's Office said: “This is a matter for Durham Constabulary and it would be inappropriate to comment while their investigation is ongoing.”
Six people were killed on June 18 1994 when loyalist gunmen burst into a bar in Loughinisland, Co Down, and opened fire on customers.
The UVF terrorists struck as football fans watched the Republic of Ireland team play in the 1994 Fifa World Cup.
In 2011, the Police Ombudsman found there had been major failings in the police investigation following the shootings, but said there was no evidence that police had colluded with the UVF.
However in 2016, a new Ombudsman report found there had been collusion, and the police investigation had been undermined by a desire to protect informers.
In 2017 the No Stone Unturned documentary named the main suspects.