A 65-year-old man is continuing to be held in connection with the murders of 21 people in the 1974 Birmingham pub bombings.
The arrest on Wednesday came just days before the 46th anniversary of the two deadly November 21 blasts which ripped apart the Mulberry Bush and Tavern in the Town pubs.
The individual in custody is understood to be Michael Patrick Reilly, who was arrested at an address in Belfast on Wednesday morning by officers from West Midlands Police assisted by the Police Service of Northern Ireland (PSNI).
He was arrested under the Terrorism Act and taken to Musgrave Street PSNI custody block in the city, while searches continued at his address throughout the day.
The force said he would be interviewed under caution at a police station in Northern Ireland.
Officers have an initial 48 hours to question a suspect under the Act, police said on Thursday.
At an inquest into the bombings last year, a jury concluded a botched IRA warning call on the night led to 21 people being unlawfully killed.
The bungled West Midlands Police inquiry in the immediate aftermath of the bombings led to the wrongful convictions of the Birmingham Six, one of the worst miscarriages of justice in legal history.
They were later freed in 1991 after their convictions were quashed.
During evidence given at last year’s inquest, an anonymous IRA volunteer named the men he said had been involved in the attacks.
The individual – identified at the hearing only as Witness O – said those who took part were Mick Murray, Seamus McLoughlin and James Francis Gavin, as well as a fourth man, Michael Hayes, who now lives in Dublin.
McLoughlin, who the witness named as the OC (officer commanding) of the Birmingham IRA at the time, died in 2014, Murray in 1999 and Gavin in 2002.
But when lawyers for the victims’ families put Mr Reilly’s name to the ex-IRA volunteer, he denied any knowledge of his involvement.
Mr Hayes has always strongly denied having anything to do with the pub bombings but has said previously he took “collective responsibility” for IRA bomb attacks.
In 2017, he told the BBC how he had defused a third bomb in Birmingham that night, after being “horrified” at the “unintended” carnage of the pub blasts.
Mr Reilly was approached for a comment on the pub bombings for an ITV documentary in 2018 by journalist John Ware.
In a statement issued at the time, Mr Reilly’s lawyer Padraig O Muirigh strongly denied allegations he was involved as being “without any foundation”.
Speaking then, Mr O Muirigh said: “These allegations are untrue and without any foundation. The naming of my client in this manner is akin to trial by media.
“My client has not been convicted of any offence in relation to the 1974 pub bombings.
“The best place for these serious allegations to be tested is within the criminal justice system and not via a television programme.
“My client is also very concerned that the identification of him in this manner has potential implications for the safety of both him and his family.”
The arrest has come a month after Home Secretary Priti Patel said she would consider holding a public inquiry into the bombings.
Ms Patel also said she wants to visit Birmingham to meet justice campaigners, including Julie Hambleton, whose 18-year-old sister Maxine died in the bombings.
Reacting to news of the arrest – the first in four decades in connection with the bombings – Ms Hambleton, who is part of the victims’ families’ campaign group Justice 4 the 21, called it “monumental”.
She broke down in tears when telephoned by a police officer with the news.
“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.”