By Michael McHugh and David Young
A police state-style arrest operation was launched against two journalists after confidential material was aired in a documentary, a barrister said.
A vast amount of unrelated research, including millions of pages of documents, was taken when officers raided offices and the homes of award-winning film makers Trevor Birney and Barry McCaffrey, their lawyers added.
The pair were detained last year over the alleged theft of a police watchdog document that appeared in their film No Stone Unturned about the murders of six men in Loughinisland, Co Down, in 1994.
They remain under live police investigation and are on bail.
Conservative MP and former Brexit secretary David Davis was among those supporting the investigative duo and said the case should be highlighted around the world.
Opening a legal case in Belfast on Tuesday morning, Barry Macdonald QC said: "This was the kind of operation more associated with a police state than with a liberal democracy."
He said investigative journalists and their sources were supposed to be protected from this kind of intrusion.
He added: "This operation sets off alarm bells and flashing blue lights amongst media organisations.
"It set off alarm bells because of the grave implications for freedom of the press."
The 2017 film broke new ground by naming suspects it said were involved in the Ulster Volunteer Force (UVF) killings of six Catholic men gathered in a village pub watching the Republic of Ireland play a World Cup football match on TV.
Mr Birney and Mr McCaffrey are taking a judicial review against the execution of a police search warrant.
Journalistic documents, computers, notebooks, files and digital material seized by police when they raided their film production company - Fine Point - were bagged and sealed after lawyers secured an interim injunction preventing detectives examining them pending the hearing of the legal challenge.
Police are investigating how information contained in a Police Ombudsman for Northern Ireland document appeared in No Stone Unturned.
Police Service of Northern Ireland (PSNI) chief constable George Hamilton, citing a potential conflict of interest, asked Durham Constabulary to take on the probe in the wake of the film's release.
No one has been convicted of the Loughinisland murders.
In a 2016 report, Ombudsman Dr Michael Maguire concluded the security forces colluded with the UVF killers.
Officers appeared to search every desk in the journalists' office and take material unrelated to Loughinisland, including one surrounding alleged clerical child sex abuse, Mr Macdonald told the Court of Appeal in Belfast.
He said the judge who granted the search warrant did not have issues surrounding the protection of journalistic sources properly in mind.
"The judge effectively unfortunately allowed himself to be used by a single-minded police officer who had his own agenda."
He added: "Vast amounts of material which were taken with nothing to do with Loughinisland."
He claimed there was no risk to suspects named in the documentary since their names had been in the public domain for years and they had refused to take up an offer from filmmakers to respond.
Outside court, Mr Davis said: "Press freedom is the most fundamental freedom in modern society because it protects all the other freedoms.
"Protecting press freedom is not just about protecting journalists, it is about protecting their whistleblowers and their witnesses, it is about protecting the ability to expose wrongdoing and expose failure and that is what these journalists are doing and they don't deserve to go through what they are going through now.
"Their cause is so important it should be known throughout the whole of the UK and the rest of the world."