A man being held in connection with the 1974 Birmingham pub bombings investigation is expected to face further questioning today.
The 65-year-old, 19 at the time of the blast, was arrested at his home yesterday and spent the night in custody.
A source said the move was at the request of West Midlands Police.
Searches were carried out at the pensioner’s property in West Belfast.
Twenty one people died in the attacks on the Mulberry Bush and the Tavern in the Town on November 21, 1974.
Another 182 were reported to have survived injuries. No warnings of the attacks were given.
News of the arrest came as a shock to campaigners.
Julie Hambleton, a leading member of Justice For The 21 – who lost her sister Maxine in the bombing said: “When I got the call from the police I thought it was about the anniversary this weekend.
“When I was told the news, I just broke down. I couldn’t breathe, I looked straight at the picture of Maxine and thought, ‘today is for you and the other 20’.
“But whilst it is a huge significant development it is just an arrest. This is still in its very early stages.
“We’re hoping this leads somewhere and we finally make some progress. This does not in any way lessen our desire for a full public inquiry.”
A large police presence was seen in the area before the man was arrested. He was permitted to dress before being taken to a city centre police station.
Six men, Hugh Callaghan, Paddy Hill, Gerard Hunter, Richard McIlkenny, William Power and John Walker, were wrongly jailed for life in 1975 for the bombings.
They were released in 1991.
A West Midlands Police source said: “The Police Service of Northern Ireland acted this morning at the request of the West Mid-lands investigation team.
“Emotions are running high but everything must be done by the book.”
West Midlands Police chief constable Dave Thompson said last year that an active investigation into the outrage was ongoing.
He said: “IRA terrorists planted these bombs. They are responsible for the atrocity.
“There are enormous challenges but I remain determined to do all we can.”