Three men have been arrested following the shooting of a long-serving prison officer in Northern Ireland.
David Black, 52, was gunned down on his way to work in a motorway ambush on Thursday.
Police have blamed dissident republicans for the attack.
On Friday morning, two men aged 31 and 44, including prominent dissident republican Colin Duffy were arrested by the PSNI in connection with the attack.
The third man, aged 29, was detained in the the Irish Republic on Friday afternoon and he is being questioned at Carrick-on-Shannon Garda station.
Report from Mark Mallet in Belfast
David Black, who was married with a son and a daughter, was the 30th prison officer killed in Northern Ireland since 1974.
Earlier, Sue McAllister, director-general of the Prison Service in Northern Ireland, his grieving widow, Yvonne, at her home in Cookstown, Co Tyrone.
The family is completely shattered. They are very close. It is a dreadful tragedy and we will make every effort to support them in whatever way we can.
The charity Crimestoppers has offered a reward of up to £10,000 for information leading to the arrest and conviction of those responsible for the murder of Mr Black.
PSNI Assistant Chief Constable, Drew Harris, welcomed the reward:
Detectives are conducting an extensive investigation into David Black's murder. A key element of this investigation will be information which is provided by members of our community. If the reward encourages some individuals to come forward, this will be of benefit to all our efforts as we work to secure convictions against Mr Black's killers."
In an urgent statement to MPs, Northern Ireland Secretary Theresa Villiers sent her "deepest condolences" to David Black's widow Yvonne, son Kyle and daughter Kyra for their "devastating loss" and vowed to catch his killers.
Mrs Villiers said: "The Government will do whatever we can to help the PSNI bring the perpetrators of this atrocity to justice.
"We will continue to do everything in our power to ensure the terrorists do not succeed."
Mr Black's killing has prompted condemnation from across Britain and Ireland.
The Northern Ireland Executive and the Irish Republic's government discussed Mr Black's murder at a North South Ministerial Council meeting.
Speaking after the meeting, Irish prime minister Taoiseach Enda Kenny said:
We condemn this murder unreservedly. "We do not want to see a return to this, nobody on this island wants to see a return to this.
Northern Ireland's deputy First Minister Martin McGuinness said supporters of dissident republicans need to "get real" and realise they are a "ragbag" of criminals out for their own ends.
Stormont First Minister Peter Robinson branded the dissident republicans who killed David Black as the enemies of everyone across the island of Ireland.
When someone strikes at the Prison Service in Northern Ireland, that's our Prison Service, that's part of our community, they are attacking us as part of that community and the community must give a community response to that.