Police have confirmed they have now launched a murder investigation following the death of Adrian Ismay and subsequent post mortem examination.
A PSNI spokesman said in a statement that the PM “concluded that Mr Ismay died as a direct result of the injuries sustained during the explosion of the under car device on his vehicle on Friday 4 March”.
Dissident republican group the New IRA, which opposes the peace process, said it was responsible.
The 52-year-old father-of-three suffered leg injuries and was said to be recovering well from surgery. But he taken back into hospital on Tuesday morning and died.
One man has already been charged with attempted murder.
Senior Investigating Officer Detective Chief Inspector Richard Campbell said: “Today is yet another difficult day for the Ismay family, his friends and colleagues as they struggle to come to terms with the events of the past 12 days. We have spoken to the family this morning and advised them of this development.
“We are also liaising with the Public Prosecution Service in relation to the individual who is currently charged with attempted murder and causing an explosion with intent to endanger life.”
DCI Campbell added: “This has been treated as a significant Serious Crime investigation from the outset and will continue as such.
“I would reiterate my appeal from yesterday about the two vehicles we believe were involved in this murder. The cars were a red Citroen C3 with registration SKZ6662 and a silver Skoda Fabia with registration KFZ2352. Anyone with information should contact detectives on the Non-Emergency 101 number or anonymously through the Crimestoppers Charity 0800 555 111”.
Mr Ismay, a veteran officer who had more than 28 years service with the Northern Ireland Prison Service, was married and had three grown-up girls.
He trained other guards at HMP Maghaberry near Lisburn and was based at Hydebank Wood Young Offenders Centre in south Belfast.
The attack happened in the Hillsborough Drive area, off Woodstock Road, a predominantly loyalist area in the east of the city, just after 7am on Friday March 4.
The New IRA claimed to have used the plastic explosive Semtex and a commercial detonator, but police have released no details.
Police have renewed appeals for information about two vehicles they believe were involved in the murder.
A red Citroen C3 - registration SKZ 6662 - is believed to have been used by those planting the device during the early hours of the morning.
A silver Skoda Fabia, with a KFZ 2352 number plate, is suspected to have been used before and after the incident by those involved.
Earlier, Sue McAllister, director general of the Northern Ireland Prison Service, said she would not be shocked by claims some paramilitary inmates at Maghaberry high-security jail had cheered, smoked cigars and mocked warders on hearing of the death.
Speaking on BBC Radio Ulster, she said: “That type of thing would not be surprising. But it would be inappropriate to comment until I have validated that for myself.
“Clearly, that sort of behaviour is completely unacceptable and it is in stark contrast to the quiet bravery and courage that is being shown by Adrian’s family at this very difficult time.
“We know that a number of prisoners in one part of Maghaberry demonstrate challenging behaviour on a daily basis towards our staff, so this would be something we are quite used to dealing with and we will deal with it appropriately.
“Prisoners do have access to tobacco through the tuck shop but until I have spoken to somebody who actually saw that, it would be too early to comment further.”
Ms McAllister has requested an updated assessment of the level of threat posed by dissident republicans, which has been severe for some time.
Following the attack on Mr Ismay, police commanders warned violent dissidents were trying to escalate their activities to mark the centenary of the 1916 Easter Rising - a pivotal date in the republican calendar which sparked a series of events that led to 26 counties in Ireland gaining independence from Britain.
Stormont Justice Minister David Ford said: “I am shocked to learn Adrian has died as a direct result of the actions of people who have obviously little regard for the lives of those who serve the community.
“This is shocking news and will be even more painful for Adrian’s family and prison service colleagues.”