The head of the Northern Ireland Prison Service said a "very serious mistake" was made when a 'dangerous' murder suspect was mistakenly released from a high-security prison.
Ronnie Armour apologised to the family of a murder victim after Michael Smith was allowed to leave Maghaberry Prison to attend a family event on Tuesday.
The Prison Service is under pressure to explain why he was released and why it took a day before the public were warned.
A hunt was under way to find Michael Smith, who police said has an "extensive history of violence".
The west Belfast man was wrongly freed from Maghaberry Prison on Tuesday and was last seen at around 12.30pm that day in the Finaghy area of the city.
Smith had been refused bail in the High Court on Monday when he applied for compassionate release to attend his son's Confirmation ceremony.
Mr Armour told the BBC's Good Morning Ulster programme he was sorry for the additional anxiety the mistake had caused Mr Carson's family.
He also apologised to the public.
"It is the role of the prison service to protect the public by holding securely those committed to us by the courts, and in this case we have failed in that responsibility," he said.
Smith, 38, of Ardmonagh Gardens, is one of two men charged with the shotgun murder of Stephen Carson (28) in south Belfast in February last year.
Smith is alleged to have fired the fatal shot after men broke into a house at Walmer Street while he was eating dinner with his partner and nine-year-old son.
The bail hearing was told that the killing was linked to a samurai sword attack which hospitalised him in 2010.
One of the men alleged to have been involved in the sword attack, Kieran McManus, was shot dead three years later in west Belfast.
Bail was refused after a judge identified a possible risk of interfering with a key witness in the Carson case.
Detective Superintendent Kevin Geddes last night said police had strenuously objected to bail and Smith was "released erroneously by the Prison Service".
He pleaded with him to "give yourself up".
The senior officer also asked family and friends to either "persuade him to give himself up" or call the police and "tell us where he is".
"He has an extensive history of violence and I would urge the public not to approach him but contact police immediately," he said.
SDLP assembly member Pat Catney said the mistaken release was "an extremely troubling development that will cause grave concern for people in the local area and beyond".
"Serious questions have to be asked as to how this happened and why it took a day before a public alarm was raised and I will be making representations to the Prison Service to get answers," he said.
The Prison Service said only that matters "surrounding the erroneous release of Michael Lawrence Smith are currently under investigation".
Hundreds of inmates have gone missing from the north's prisons over the past decade.
Between 2007 and late 2015, prisoners went on the run more than 200 times, with some remaining at large for several years.
In most cases inmates failed to return after taking part in various types of temporary release schemes.
Of the 218 incidents of prisoners being unlawfully at large, 95 involved Magilligan medium-security jail, 93 were inmates at Maghaberry high-security prison and 30 at Hydebank, which houses female prisoners and young offenders.
The figures, released to news website The Detail, showed that the majority failed to return while on home leave. In most cases it was for a few days, but one prisoner went missing from Maghaberry in 2009 for four years.
According to the Department of Justice website, there were last night a total of six inmates unlawfully at large including murder suspect Michael Lawrence Smith.
His disappearance comes after a man went missing for months while on bail charged in connection with the 2012 murder of prison officer David Black.
Damien McLaughlin (40), from the Ardboe area of Co Tyrone, was arrested last week in Co Donegal. He was granted bail in May 2014 but had not been seen by police since last November.
The release of Michael Smith is not the first time someone has been let out of jail by mistake.
On four occasions between 2010 and 2015, prisoners were released early – two in 2010 who were on remand, and one in 2012 while being held on an immigration order.
The fourth was mistakenly released early from Maghaberry in 2011 while serving a six-month sentence for assault and possession of a dangerous weapon.
Prison chiefs have also faced criticism in recent years for using dated photographs of on-the-run prisoners during appeals for information on their whereabouts.
In 2015 a picture of Thomas Valliday, who murdered west Belfast father-of-two Frank 'Bap' McGreevy in 2008, was released after he escaped from prison staff during a hospital visit.
But Mr McGreevy's family branded the appeal a "joke" after authorities used a seven-year-old photo of the Maghaberry life-sentence prisoner.
The Department of Justice released a more recent photo after being contacted by The Irish News and issued an apology. Valliday was later arrested in Belfast after four days on the run.
