Two Northern Irish journalists arrested over the alleged theft of confidential documents relating to the Loughinisland massacre were told their bail would only be extended if they agreed not to talk about aspects of the case.

Amnesty International said the treatment of Trevor Birney and Barry McCaffrey meant freedom of the press in Northern Ireland was at risk, while the National Union of Journalists said the police had violated “basic media freedoms” amid growing international concern about the case.

Police raided the journalists’ homes last summer after they helped make the documentary No Stone Unturned, which re-examined the murder of six people in County Down in 1994. Three masked gunmen carried out the raid on a packed pub during a World Cup match between the Republic of Ireland and Italy, but no one has ever been charged, despite claims that the key suspects are known to the police.

Birney and McCaffrey worked with the Oscar winner Alex Gibney to produce the film about the massacre, which included previously secret evidence from an unredacted internal police report, named alleged killers and hinted at collusion between the authorities and the Ulster Volunteer Force killers in the incident.

The journalists urged police to pursue the killers and said the report, the accuracy of which has not been disputed, had been leaked to them by an anonymous source. Instead, about 100 police officers were deployed last summer to raid the journalists’ homes and offices and the pair were arrested on suspicion of stealing the material.

On Friday, the police unsuccessfully attempted to attach a condition to the extension of the pair’s bail which would stop them talking about aspects of the case.

“The police are trying to attack press freedom by imposing draconian bail conditions,” said Birney shortly after leaving the police station. “They are trying to close the door after the horse has bolted because they have realised how contemptible their action has been. There is an attempt to gag us from bringing the full details into the public domain. Why are they concerned about us being able to speak publicly about this case?”

He claimed the Police Service of Northern Ireland was attempting to protect the reputation of individuals involved in the Royal Ulster Constabulary, its predecessor force, and called on Westminster to take an interest in the case.

“It’s highly ironic that the Foreign Office is investing in issues about press freedom around the world, but once again the British government is showing a misunderstanding about what’s happening in Northern Ireland. The British government is allowing one of its own British constabularies to arrest journalists. It’s out of sight, out of mind.”

The scene inside O’Toole’s Bar after the killings. Photograph: PA

Durham constabulary has been called in as an external police force to lead the investigation. The force insisted that the proposed bail condition was an attempt to stop the journalists discussing witness statements disclosed to them as part of the legal process surrounding the incident, rather than a blanket ban on talking about the case.

“At no stage today did Durham ask for a condition stopping either person talking about their arrest, as has been suggested,” said a spokesperson.“Today’s process is not an attempt at gagging anyone. We simply would not wish to see our investigation undermined by having witness statements inappropriately disclosed on social media.”

Birney, backed by his solicitor, dismissed the police’s account of events: “That is completely untrue. That is not what happened in the police station. That is complete revisionism.”

The journalists, who have been bailed until September, have to give the police three days’ notice of their intention to cross any national border. Birney said the prospect of a year on bail without charge was placing a strain on his family life. “Police are operating without proper political oversight. I don’t believe this would happen in London. Because it’s Belfast, it seems to be OK. This is an attack on the freedom of journalists operating inside the UK.”

The NUJ described the police’s actions as a “blatant attempt to thwart the massive international campaign against the arrest of two journalists whose only crime is their search for truth and justice”.

“These journalists are being punished because they have exposed brutal human rights abuses in Northern Ireland,” said Séamus Dooley, the NUJ’s assistant general secretary for Ireland. “The legal threats, harassment and intimidation must stop. A free press is critical to the health of democracy, and freedom of expression is a fundamental human right. The police should not be allowed to continue to violate basic media freedoms.”

Despite the potential prosecution, No Stone Unturned is still available to purchase on online video services such as iTunes and Amazon Prime Video, and it is hoped that it will be shown on television in the UK this year.