Ten Protestant workmen were killed in the massacre
A 59-year-old man has been arrested by detectives investigating the sectarian murder of ten Protestant workmen at Kingsmill in Northern Ireland in 1976.
The Police Service of Northern Ireland say the arrest also relates to the attempted murder of one other man who survived the attack.
The workmen were shot dead after their van was stopped near the village of Kingsmill.
They were travelling home from work in a textile factory when they were ambushed.
Detective chief inspector Ian Harrison of the Legacy Investigation Branch said the man was arrested in Newry, and is assisting with enquiries.
The PSNI are also appealing for anyone with information to contact them.
The IRA has been widely blamed for the mass shooting, which was seen as a reprisal for loyalist killings in the same area.
Two months ago, police revealed a potential match had been found to a palm print left on a getaway vehicle.
DUP MLA for Newry and Armagh, William Irwin, has said the families deserve justice.
Mr Irwin said: "40 years have passed since one of the worst sectarian atrocities of the Troubles. The pain of the Kingsmills families has not diminished over those decades and they deserve to see someone brought to justice for the murder of their loved ones."
"There is obviously an active criminal case which must be taken forward and I would hope that can be taken forward swiftly."
"The Kingsmills atrocity was not planned and carried out by one individual alone however and it is vital that whilst the possibility of justice remains that all efforts should be expended to identify all those involved," he added.
Survivor Alan Black met police investigating a new lead in the case in June.
"It has to be done right - quick is great, but it has to be done right," Black told UTV back in June
"And if that means a bit more time then so be it," he added.
Mr Black, who was shot 18 times, said the development has come like a "bolt from the blue".
"I don't really know what to think to be honest. I am still trying to take it in," he said in a statement released by ITV.
"For 40 years the police were not interested in Kingsmill but this, I suppose, is a development at least," he added.
Detectives investigating murders at Kingsmill in 1976 have arrested a 59 yo man on suspicion of murder of 10 men and att murder of 1 man.— PSNI (@PoliceServiceNI) August 5, 2016