A man has been arrested at Belfast International Airport in connection with a 1990 Co Armagh bomb.
The 54-year-old man has been arrested by detectives from PSNI Legacy Investigation Branch.
Detective Inspector Deborah Eakin said: “The male was detained at Belfast International Airport this morning on suspicion of a number of offences including the murder of three police officers and a Catholic nun in an explosion on the Killylea Road in Armagh on 24 July 1990.
“He has been taken to the Serious Crime Suite at Musgrave Police Station in Belfast for questioning.”
Constable William Hanson, Reserve Constable Joshua Willis and Reserve Constable David Sterrit were killed in the blast.
Sister Catherine Dunne was also killed while another woman was also seriously injured in the blast.
At the time it was reported that the three uniformed members of the RUC were killed instantly when their unmarked patrol car ran over and detonated an explosive device, two miles south of Armagh.
The blast lifted the car over a high hedge into a field, where it landed on its roof.
The nun and another woman were in a car going in the opposite direction when they were also caught by the blast.
The bomb, believed to have contained at least 1,000 pounds of explosives, left a 20-foot crater in the road.
A 55-year-old man was also arrested and questioned last week in connection with this incident.