Prison officers are demanding the sudden death of their bomb victim colleague Adrian Ismay is treated as murder by police.
The 52-year-old died from a heart attack yesterday morning, less than two weeks after he survived a terrorist bomb attack.
It has emerged the married father-of-three was due to become a grandfather, with one of his three daughters about to give birth.
Some republican inmates at Maghaberry Prison reportedly celebrated the death by taunting warders and smoking cigars.
Detectives are awaiting the results of medical tests to see if Mr Ismay's heart attack was related to injuries he sustained when a booby-trap device exploded under his van in east Belfast on the morning of Friday, March 4.
Fellow prison officers insisted Mr Ismay's death could not be treated as anything other than murder. "They placed a bomb under his van and days later he died of a heart attack," one said. "It'd be a disgrace if police treat this as attempted murder. What sort of signal would that give out to those groups targeting us?"
Sean McCarry, a close friend of Mr Ismay who worked with him in the Community Rescue Service which searches for missing people, called him a "decent man".
He said his friend had told him he planned to retire from the Prison Service and volunteer for the charity on a full-time basis following the attempt on his life.
"Adrian didn't allow his injuries and what happened to make him bitter," he added. "He saw it as an opportunity for him to move on and help others."
First Minister Arlene Foster said she was "personally devastated" by news of Mr Ismay's death. She tweeted: "Can't believe the news. I was texting Adrian before we left for the US. He was doing well. Thoughts with his family."
Deputy First Minister Martin McGuinness added: "I am deeply saddened that this prison officer has lost his life. Thoughts and prayers with his wife and three daughters. These are futile acts which achieve nothing."
Justice Minister David Ford praised Mr Ismay's courage, revealing he attended a passing out parade for new recruits to the service last Friday, just a week after he was attacked. He said this contrasted with the cowardice of those who planted a bomb under his van in the dark of night.
Unionist politicians joined calls for Mr Ismay's death to be treated as murder. Ulster Unionist Party leader Mike Nesbitt said: "Whatever the exact cause of death, I cannot accept he would be dead if it were not for the bomb."
DUP MLA Paul Givan added: "Adrian's colleagues see this as murder. A bomb was placed under his vehicle. The intent was to murder. He had a heart attack within days. Most people will be expecting that this will be a murder investigation."
Police yesterday appealed for information about a red Citroen C3, registration SKZ 6662, which is believed to have been used by those who planted the device, in the Hillsborough Drive area of east Belfast at around 2.20am on the day the bomb exploded.
They are also looking for information about a silver Skoda Fabia, KFZ 2352, which is believed to have been used in the attack. In particular, detectives would like to hear information about any sightings of the car between 7pm on Thursday, March 3 and 4am on Friday, March 4.
The senior investigating officer, Detective Chief Inspector Richard Campbell, said police believe a male was dropped off in the Skoda Fabia in Pilot Street in the Docks area of Belfast at around 3am on March 4. He asked anyone who saw this, or who knows the identity of the man, to come forward to police.
Mr Ismay was a long-serving officer based at Hydebank Wood Young Offenders Centre in south Belfast. He worked as a trainer for new Prison Service recruits and had served for 28 years.
He was released from hospital last week, where he was treated for injuries sustained during the bomb attack, but he was rushed back yesterday morning after suffering a heart attack.
A dissident republican group calling itself the new IRA claimed responsibility for the attack. It said that Mr Ismay was one of a number on a list of potential targets and that the attack arose from a dispute over the treatment of dissident republican inmates in Maghaberry.
Last week, Christopher Robinson (45) from Dunmurry, Co Antrim, appeared at Belfast Magistrates Court charged with the attempted murder. The defendant was also charged with possessing an improvised explosive device with intent to endanger life.
Mr Ismay is the second prison officer to die following an attack by dissidents, after David Black, who was shot dead on the M1.
Deputy Chief Constable Drew Harris said more attacks were planned ahead of the 1916 Easter Rising commemorations, and Police Federation chairman Mark Lindsay warned those in the security forces "to exercise maximum vigilance at this dangerous time".
The feelings of outrage and anger that gripped the Cregagh Road area of Belfast after the dissident republican bomb attack on a prison officer 12 days ago were compounded yesterday by a sense of shock after the victim, who was due to become a grandfather, died from a suspected heart attack.