Last month officers from the Durham Constabulary arrested two investigative journalists, Trevor Birney and Barry McCaffrey, in Belfast on suspicion of using material about the Loughinisland massacre, allegedly stolen from the Police Ombudsman for Northern Ireland.
Alex Gibney, whose TV documentary the pair had worked on, said the police should instead have reopened the investigation into the 1994 massacre.
But what happened at Loughinisland and how many other massacres from "The Troubles" remain unsolved?
Loughinisland
Mr. Gibney's documentary No Stone Unturned begins with a reconstruction of the night of June 18, 1994 when three masked men entered The Heights pub in Loughinisland, a largely Irish nationalist village in County Down.
Ireland won the match 1-0 but five minutes after Ray Houghton's goal went in the gunmen entered The Heights and shot 11 men in the back.
The oldest of the victims was 87-year-old Barney Greene.
The gunmen, believed to be an Ulster Volunteer Force unit from loyalist east Belfast, were armed with a Czech-made VS58 rifle, similar to an AK-47.
They laughed as they ran away and got into a car, which was found abandoned near Ballynahinch.
The UVF later claimed the victims were attending an Irish "republican function", a claim which was dismissed by police.
Both the Pope and the Queen sent messages of sympathy to the victims.
Nobody has ever been convicted of involvement in the massacre despite claims made in the documentary that the identity of three men was known to the police at the time — one of whom was a former British Army soldier who lived locally.
The Provisional IRA declared a ceasefire two months after the Loughinisland massacre.
Ballymurphy
Another massacre which has recently been in the headlines is the killing of a Catholic priest, a mother-of-eight and eight other people were shot dead by members of the British Army's Parachute Regiment in the Ballymurphy district of west Belfast over three days in August 1971.
At the weekend Channel Four screened a documentary, Massacre At Ballymurphy, which contained interviews with many eyewitnesses and relatives of the victims.
But journalist and author Malachi O'Doherty criticized it for having "very little contextualisation" and ignoring the violence perpetrated by the IRA at the time.
In 2016 Sir Declan Morgan, the Lord Chief Justice of Northern Ireland, ordered a new inquest into the deaths, which is due to open in Belfast in November.
Kingsmill
On the night of January 5, 1976, an IRA gang flagged down a minibus near the village of Kingsmill in County Armagh, close to the border with the Irish Republic.
The minibus contained 12 textile workers — one of them survived after being identified as a Catholic and told to run away.
Eleven others, all Protestants, died. Alan Black survived, despite being shot 18 times.
The killings, which were in response to several sectarian killings by the UVF, were carried out by the IRA using a cover name — the South Armagh Republican Action Force.
An investigation in 2011 by the Police Service of Northern Ireland's Historical Enquiries Team (HET) said the "calculated slaughter" was "purely sectarian", which was why the IRA had sought to hide behind a cover name.
Nobody has ever been convicted for the Kingsmill massacre but the killings remain a potent symbol of The Troubles.
In January the Sinn Fein MP for West Tyrone, Barry McElduff, was forced to resign after tweeting a video on the anniversary of the massacre of him clowning around with a loaf of Kingsmill bread on his head. He was replaced by 26-year-old Órfhlaith Begley, who won a by-election in May.
Sean Graham Betting Shop
The Lower Ormeau Road in north Belfast is a predominantly Catholic part of the city so it was safe to assume most of those congregated inside a branch of the betting shop chain Sean Graham there would be Catholics.
It was the ultimate soft target.
At 2.20pm on February 5, 1992 two loyalist gunmen burst in and sprayed 44 bullets at the customers, who were waiting to bet on horses or greyhounds.
The UDA put out a statement which gave little justification, other than describing the Lower Ormeau Road as "one of the IRA's most active areas" but added "Remember Teebane," a reference to an IRA bombing three weeks earlier which killed six construction workers on their way to repair a British Army base at Omagh.
Nobody has ever been convicted of the murders but in 2010 the HET said a gun used in the attack had been given back to the UDA by the police.
"(This) will come as no surprise to the people of the Lower Ormeau area who have long known that a high degree of collusion took place in this attack," said veteran Sinn Fein politician Alex Maskey at the time.
Bloody Sunday
The most infamous massacre of them all — U2 wrote a hit song about it — took place in 1972 in Londonderry.
On January 30, 1972, around 10,000 Irish Catholics marched from the Bogside district of the city — which they referred to as Derry — demanding an end to discrimination when it came to jobs and housing.
What happened next has always been disputed with soldiers claiming they came under fire from IRA gunmen.
But what is known is that 28 unarmed civilians were shot, 13 of whom died almost immediately and a 14th died of his injuries the following year.
One of the iconic images of the day was a Catholic priest waving a blood-stained white handkerchief as he tried to secure safe passage for 17-year-old Jackie Duddy, who was fatally wounded.
A public inquiry, by Lord Widgery, cleared the army of blame but was later widely discredited.
When Tony Blair came to power in 1997 he ordered a new investigation but the Saville Inquiry took far longer than expected and did not submit its report until June 2010.
Its main finding was that those who died were all innocent and the Police Service of Northern Ireland launched a murder investigation as a result.
Eighteen ex-paratroopers have been reported to the Public Prosecution Service in connection with the killings but nobody has yet been charged.
On Tuesday, September 11, the former PSNI Chief Constable, Sir Hugh Orde, questioned the wisdom of continuing the investigation.
"The chances of a realistic prosecution from a case 50 years ago is highly, highly unlikely," said Sir Hugh.
"All state killings need to be investigated; the citizens of this country need to be able to trust the rule of law, to trust the government and to know they have equality of justice," Kate Nash told a radio station.
Greysteel
One massacre which was "solved" was the killing of eight Catholics at Greysteel, near Londonderry, although the victims got little "justice".
It was a Saturday night, the day before Halloween — October 30, 1993 — when two armed men in boilersuits and balaclavas entered the Rising Sun pub at Greysteel.
"Trick or treat," one of them said and a woman in the pub, thinking it was a prank, replied: "That's not funny."
Seconds later they opened fire with an AK-47 and a Browning 9mm pistol.
Eight people — seven of them Catholics — were killed and 19 injured.
The attack was an act of revenge carried out by the Ulster Defence Association (UDA) after a botched IRA bombing a week earlier killed nine Protestants — including Michelle Baird, aged seven, and 13-year-old Leanne Murray — at a fish and chip shop.