A 75-year-old man has been arrested in connection with a UVF bomb attack which killed 15 people over 40 years ago.
Fifteen people were murdered when McGurk's Bar was blown up in north Belfast by the UVF in December 1971.
A man was arrested in east Belfast in connection with the massacre and remains in police custody.
Following a report by the Police Ombudsman in 2011, the Chief Constable undertook to review the case to identify any possible opportunities for further investigation.
That led to a review process, which has now "identified investigative potential".
The officer leading the investigation, Detective Chief Inspector Justyn Galloway, said "there remain a number of opportunities to make progress".
"We will do our best to develop these lines of enquiry as thoroughly and quickly as possible," he said.
"At the same time, I would ask those individuals who know who was involved in this attack on December, 4 1971 to talk to us.
Much has changed in the four decades since this terrible atrocity but the families of the victims and survivors continue to suffer. I would ask that anyone with information or suspicions about this bomb attack to come forward."
Anyone with information can contact police on 02891801507 or email zMIT3@psni.pnn.police.uk