LONDON — Two journalists who worked on a documentary about a mass killing in Northern Ireland were arrested Friday on suspicion of stealing confidential documents.

Police said two men, aged 51 and 48, were arrested in Belfast on Friday morning by officers investigating the alleged theft of documents from Northern Ireland's police ombudsman.

Academy Award-winning filmmaker Alex Gibney tweeted that Trevor Birney and Barry McCaffrey had been detained, calling it "outrageous."

Birney and McCaffrey worked on Gibney's 2017 documentary "No Stone Unturned" about a 1994 massacre in the village of Loughinisland, 20 miles (32 kilometers) southeast of Belfast.

Birney, a longtime newspaper and television journalist, was a producer on the movie. Investigative reporter McCaffrey also worked on the film.

Six people were killed when Protestant paramilitary gunmen burst into a bar and opened fire on customers watching Ireland play in a World Cup soccer match. No one has ever been convicted of the killings.

A 2016 report by the police ombudsman said the police investigation had been undermined by a desire to protect informers inside outlawed paramilitary groups.

"No Stone Unturned" explored the botched investigation and allegations of police collusion, and named suspects in the attack.

Clare Rogan, a spokeswoman for the victims' families, accused British authorities of trying to cover up the state's role in the murders.

"Today's arrests show the lengths of desperation that the British government and state forces are prepared to go to, in order to stifle the truth about what happened in Loughinisland," she said.