A man has been arrested on suspicion of terrorism offences in connection with the 1974 IRA Birmingham pub bombings.
The 65-year-old was arrested at his home in Belfast by officers from West Midlands police working alongside colleagues from the Police Service of Northern Ireland.
The breakthrough comes just days before the 46th anniversary of the atrocities which saw 21 people murdered when the IRA detonated bombs at the Mulberry Bush and Tavern In The Town pubs in the centre of Birmingham.
Six men, who were jailed for the attack in 1975, subsequently had their convictions quashed in 1991, after one of the most serious miscarriages of justice in British legal history was exposed.
Last year at a fresh inquest into the deaths of the 21 victims, a former member of the IRA named four men he said had been responsible for the bombings.
He told the inquest that Seamus McLoughlin, Mick Murray, James Gavin, and Michael Hayes had carried out the atrocities on the orders of the IRA.
Three of the men he named are dead and Hayes, 69, now lives in Dublin.
A fifth man was later named in a television documentary as having been involved in the attack.
The families of those who died in the bombings and who have continued to campaign for justice, welcomed the latest development.
Julie Hambleton, whose 18-year-old sister Maxine died in the bombings, said it was the most "the most monumental event" since the Birmingham Six were cleared almost 30-years ago.
Julie Hambleton, who lost her sister Maxine Hambleton, has welcomed the arrest
When she was telephoned by a senior West Midlands Police officer with news of the arrest on Wednesday, she told of how she broke down in tears.
"I couldn’t speak, I was just inconsolable and was just looking at the picture of Maxine," she said.
"It’s welcome news. It’s overwhelming news. It’s tangible progress."
Ms Hambleton, who is part of campaign group Justice for the 21, said: "It’s something we have been waiting a long time for.
"Having this development – whatever happens – does not in any way lessen our desire for a full public inquiry to be held.
"There are wider issues which need to be examined and so much that went wrong, like why six men were arrested for a crime they didn’t commit."
Ms Hambleton added: "The fact is we have had to beg and campaign and give up our lives as we knew them to fight for justice.
"Justice that was never facilitated by the authorities whose job it was to do so.
"How was it that for so long, after 21 people were blown up and more than 200 other innocent souls were injured, nobody was looking for the perpetrators?"
In April last year, an inquest jury found a botched IRA warning call led to the deaths of 21 people unlawfully killed in the atrocity.