The cars which may have been used by prison officer bombers

Mr Ismay died suddenly today following what may have been a heart attack.

Detective chief inspector Richard Campbell said they are a red Citroen C3 SKZ6662 believed to have been used by those planting the device at approximately 2.20am during the early hours of Friday, March 4.

The second is a silver Skoda Fabia KFZ2352 suspected to have been used before and after the incident by those involved

Mr Campbell said: “Adrian’s profession was simply to keep people safe and we will do everything possible to bring those responsible to justice.”

Mr Ismay was a veteran officer, a married father of three grown up girls and trained other guards at HMP Maghaberry near Lisburn. He was based at Hydebank Wood Young Offenders Centre in south Belfast and had more than 28 years service.

The attack happened in the Hillsborough Drive area, off Woodstock Road, just after 7am. The New IRA claimed to have used the plastic explosive Semtex and a commercial detonator but police have released no details.

Mr Campbell added: “This was a completely senseless attack which only serves to demonstrate the ruthlessness and recklessness of those opposed to peace and who live for violence.”

Mr Campbell said a man was dropped off in this vehicle in Pilot Street in the Docks area of Belfast at around 3am on Friday morning and appealed for anybody who saw him to come forward.

Prison Officers Association (POA) Northern Ireland chairman Finlay Spratt said morale was low.

Although the exact circumstances of the death have yet to be fully established, Mr Spratt claimed the explosion was a contributing factor.

He said: “He was not a person that I knew had any sickness or ailments.”

Detectives said they had prevented a number of murder bids since the attack.

They warned there could be more attacks by dissidents ahead of this month’s centenary of the Easter Rising, which marked Irish rebellion against Britain. Senior officers have said there are several hundred active dissidents.

The threat to members of the security forces in Northern Ireland is severe from gunmen opposed to the 1998 Good Friday peace agreement which largely ended decades of the Troubles.

Warder David Black, 52, was gunned down on the M1 motorway as he drove to work at Maghaberry high security jail in Co Antrim in November 2012.

First Minister Arlene Foster and Deputy First Minister Martin McGuinness said the latest victim was targeted by those who used the cover of darkness to try and create fear.

“We cannot and will not allow people who are wedded to the past to set the tone and direction of our shared future.”

Prime Minister David Cameron said he was deeply saddened.

Irish Taoiseach Enda Kenny said he had warned repeatedly that such attacks can have no place in a civilised, inclusive society.

“We must continue to work at all levels to copper fasten a future for Northern Ireland that is committed to the democratic process and the rule of law and is free from violence and intimidation.”