Two people have been arrested in connection with the murder of prison officer David Black on the M1 motorway in Northern Ireland.
The men, aged 31 and 44, were arrested in the Lurgan area of County Armagh and taken to the Antrim Serious Crime Suite for questioning, the Police Service of Northern Ireland (PSNI) said.
Black, 52, was shot in his car as he drove down the M1 towards his place of work, the top-security Maghaberry jail. His black Audi A4 then veered into a ditch and he died at the scene.
The PSNI believes dissident republicans who are protesting against prison conditions at Maghaberry are responsible for the killing.
Black, a father of two from Cookstown, is the 30th prison officer to be killed in Northern Ireland since 1974 and the first since 1993.
His wife Yvonne implored unionists to refrain from seeking retaliation for the murder of Black, a member of the Protestant Orange Order.
She said in a statement: "Grief and sadness in another home will achieve nothing."
The murder has been widely condemned by all political parties in Northern Ireland.
First minister Peter Robinson and deputy first minister Martin McGuinness condemned the "brutal attack", saying in a joint statement that "people who work for the Prison Service play a crucial role in our community and any attack on them is an attack on all of us.
"Actions like this have no place in society and those who carried out this murder have nothing positive to contribute.
"We refuse to let the people behind this attack divert us from building a better and peaceful future for everyone."
The Republic of Ireland's foreign minister Eamon Gilmore also expressed his condolences.
"I know that I speak for every decent man, woman and child on this island, North and South, in expressing revulsion at this act."
Ulster Unionist Party leader Mike Nesbitt said: "We offer our sincere condolences to them and hope that they gain strength to see them through the coming days.
"Those who have carried out this attack on an innocent man represent the past and have nothing to offer the men, women and children of this country who have long since rejected terrorism."
Northern Ireland Secretary Theresa Villiers said it was "a cowardly and evil attack".
"The thoughts and deepest sympathy of us all are with the family, friends and colleagues of the murdered prison officer," she said.
PSNI Chief Constable Matt Baggott said the ambush was a "completely senseless attack which demonstrates the ruthlessness and recklessness of those opposed to peace and who live for violence."
Black had spent more than 30 years in the prison service and was approaching retirement.