The 55 year old, who was held in Armagh (pictured below) yesterday morning over the blast in 1990, was freed pending a report to the Public Prosecution Service in Northern Ireland.
He was detained by detectives from the Police Service of Northern Ireland's (PSNI) Legacy Investigation Branch over the explosion which occurred on 24th July 1990.
A landmine detonated on Killylea Road, Armagh, killing Sister Catherine Dunne, Constable William Hanson, Reserve Constable Joshua Willis and Reserve Constable David Sterritt. A second woman was seriously hurt.
According to PSNI Detective Inspector Deborah Eakin, the man was held on suspicious of various offences including the four murders. He was questioned at the Serious Crime Suite at Musgrave Police Station in Belfast.