A man has been released pending a report to the PPS after being arrested this morning in connection with an IRA bomb attack that killed a Catholic nun and three police officers.
The 54-year-old suspect had been detained at Belfast International Airport.
He is the second man detained in relation to the 1990 attack in Co Armagh in less than a week.
A 55-year-old man arrested and questioned last Thursday was released pending a police file being submitted to Northern Ireland's Public Prosecution Service.
Sister Catherine Dunne, Constable William Hanson, Reserve Constable Joshua Willis and Reserve Constable David Sterritt were killed when a landmine exploded on the Killylea Road, Armagh, on July 24 1990.
Another woman was seriously injured in the blast.
The arrests have been carried out by the Police Service of Northern Ireland's legacy investigation branch.
The suspect detained on Monday has been taken to the PSNI's serious crime suite at Musgrave station in Belfast.
This evening a PSNI spokesman said the 54-year-old arrested this morning in connection with a bomb attack in Armagh in 1990 that killed three police officers and a Catholic nun, "has been released pending a report to the PPS".