The Government will do "whatever we can" to help bring prison officer David Black's killers to justice, Northern Ireland Secretary Theresa Villiers has told the House of Commons.
Mr Black was gunned down in his car on the M1 motorway near Lurgan on Thursday as he drove to work at Maghaberry Prison.
Two men, including dissident republican Colin Duffy, have been arrested over the murder.
In an urgent statement to MPs, Mrs Villiers condemned the "cowardly and evil" attack and sent her "deepest condolences" to Mr Black's widow Yvonne, son Kyle and daughter Kyra for their "devastating loss".
She said: "Our profound sympathies must go to all of his family and friends at this very difficult time, and to the Prison Service, who have lost such a valued friend and colleague."
Mr Black's car was found with "a number of bullet holes" at 7.30am on Thursday, the Northern Ireland Secretary said. She urged anyone with information to contact police, saying Police Service of Northern Ireland (PSNI) officers would not rest until the killers were convicted.
Sinn Fein president Gerry Adams said the killing of the prison officer was wrong. There was no future in such actions, which were rejected by the entire community.
He added: "Whatever position individuals may hold on the efficacy or otherwise of armed struggle in the past, there is no rationale for it in the present circumstances. There is no rationale now for the existence of armed groups or for carrying out armed actions in any part of this country. Those involved have no popular support or political strategy.
"There may be a small number of people who tolerate the existence of militarist groups or their violent actions or who provide shelter, resources or facilities to them. They need to reflect on what they are doing. These groups are not the IRA and nobody should be under any illusion about that."
Mrs Villiers added: "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."