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 on Tuesday morning and died. Police will have to await medical evidence before confirming whether his death will be treated as murder.
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."