A prison officer who died after he was injured in a bomb attack in Belfast may have had a heart attack, his colleagues said.
Adrian Ismay, 52, was hurt after a booby trapped bomb exploded under the van he was driving on March 4.
Dissident republican group the New IRA claimed responsibility.
He was said to be recovering well from surgery for severe leg injuries but was taken back into hospital this morning and died.
Police will have to await medical evidence before confirming whether his death will be treated as murder.
Taoiseach Enda Kenny extended his deepest sympathies to the family, friends and colleagues of Adrian Ismay.
Speaking from Washington, the Taoiseach said: "My thoughts and prayers are first and foremost with Adrian Ismay's family for their loss and for their trauma in the wake of the bomb attack which caused Adrian such terrible injury.
"I have said repeatedly that such attacks can have no place in a civilised, inclusive society and we must continue to work at all levels to copperfasten a future for Northern Ireland that is committed to the democratic process and the rule of law and is free from violence and intimidation."
Justice Minister David Ford and Prison Service director-general Sue McAllister said: "Adrian Ismay gave over 28 years of service to prisons in Northern Ireland and he was greatly respected by all those who knew him.
"Our thoughts and prayers are with his family, friends and colleagues at this difficult time."
The attack happened in the Hillsborough Drive area, off Woodstock Road, a predominantly loyalist area in the east of the city, just after 7am.
Mr Ismay was based at Hydebank Wood Young Offenders Centre in south Belfast, where he was a trainer for new recruits to the Northern Ireland Prison Service.
In a statement to the BBC, the New IRA said the officer was targeted because he was involved in training other guards at HMP Maghaberry, near Lisburn.
A spokesman said Mr Ismay was one of a number of people on a list of potential targets and the attack arose from a dispute over the treatment of dissident republican inmates.
One Stormont Assembly member has claimed they are trying to return Northern Ireland's high-security prison to conditions similar to the old Maze Prison, where republicans won a series of concessions and famously went on hunger strike.
The New IRA claimed to have used the plastic explosive Semtex and a commercial detonator in the attack.
First Minister Arlene Foster tweeted: "I'm devastated. Can't believe the news ... My thoughts are with his family."
Northern Ireland Secretary Theresa Villiers said she was shocked and saddened by the prison officer's death.
She tweeted: "My thoughts & deepest condolences are with his family."
Police Service of Northern Ireland (PSNI) Detective Chief Inspector Richard Campbell said: "Adrian was the father of three grown-up daughters and had over 28 years' service with the Prison Service. Our deepest sympathy is with Adrian's family, friends and colleagues at this sad time.
"One man has been charged with attempted murder and causing an explosion with intent to endanger life.
"The investigation is continuing. At this stage, we are working to establish the exact cause of Adrian's death."
Ulster Unionist leader Mike Nesbitt said: "I am devastated to hear of the death of prison officer Adrian Ismay.
"My thoughts are with his family and friends and all who put on a uniform to keep us safe. Whatever the exact cause of death, I cannot accept he would be dead if it were not for the bomb."
In a joint statement, Mrs Foster and Deputy First Minister Martin McGuinness, who are in Washington, said those responsible for the bomb attack were wedded to the past.
The statement said: "This is devastating news for the family who have lost a much-loved husband and father. Our prayers are with them all at this incredibly difficult time.
"Although we are far from home, foremost in our thoughts are a grieving family. A family that was unashamedly targeted by those who used the cover of darkness to inflict terror and try to create fear in our community.
"We cannot and will not allow people who are wedded to the past to set the tone and direction of our shared future.
"Later today we are meeting with President Obama safe in the knowledge America will continue to stand strong with us to face down and defeat violence regardless of whatever quarter it comes from."
Minister for Foreign Affairs Charlie Flanagan said that the attack was a callous and cowardly one on the entire community.
In a statement this afternoon, Minister Flanagan offered his condolences to Adrian's family, friends and colleagues.
He went on to say that everyone must work together to reject those who wish to return Northern Ireland to the days when these heinous attacks were commonplace.