The investigation: Two arrested as Sinn Fein brands gunmen 'traitors'

A man and a teenager were being questioned last night by detectives investigating the murder of a police officer shot dead by dissident republicans in Northern Ireland on Monday night.

The arrests of the 37-year-old man and the 17-year-old came as Martin McGuinness delivered Sinn Fein's strongest-ever condemnation of dissidents, describing the gunmen as "traitors to the island of Ireland".

The officer who was shot dead by the Continuity IRA (CIRA) splinter group has been named as Constable Stephen Paul Carroll, 48. The two people under arrest are being held at a specialised police questioning centre near Belfast. During the day houses were searched in the area of Craigavon, Co Armagh, where the attack took place. The suspects can be held for several days without charge under anti-terrorist legislation.

PC Carroll was one of a number of officers who went to the Lismore Manor housing development in two cars in response to a call from a distressed woman. Two neighbourhood officers went into a house to investigate while the other three waited. They were then ambushed by a gunman who emerged from the darkness and fired two shots. As he arrived at the scene, PC Carroll was shot through the rear window of his car.

The incident produced a renewal of the political and community outcry which followed Saturday night's killing, also by dissidents, of two British soldiers outside their military base at Antrim. That attack was the work of the Real IRA, while the murder of the policeman was carried out by a similar but separate group, the CIRA.

It is not yet clear whether there was co-ordination between the two organisations. Some time ago, the Chief Constable of Northern Ireland, Sir Hugh Orde, warned that there was "probably an almost appalling competition" between dissident groups to kill a police officer.

PC Carroll was just two years from retirement after serving 24 years as a policeman in Northern Ireland during some of the most violent spells of the Troubles. A Catholic, he lived in Banbridge, Co Down, with his wife and stepson. He also had three grandchildren. His wife, Kate, said yesterday: "A good husband has been taken away from me and my life has been destroyed. And what for? A piece of land that my husband is only going to get six feet of. These people have just taken my life as well."

The family's priest, Canon Liam Stevenson, said of the family: "They had plans and dreams for the future and now all this has been blown apart."

The family was visited by the Northern Ireland First Minister, Peter Robinson, and Mr McGuinness, the Deputy First Minister, who had twice delayed a visit to the United States. Sir Hugh also saw the family.

Mr Robinson and Mr McGuinness have been commended for what is regarded as a remarkable show of political unity in the face of the violence.

Mr McGuinness said: "As I call on the people, our people, to wholeheartedly weigh in behind the police service north and south in the apprehension of these people, I have to make it absolutely clear that I too have a duty and a responsibility, if I knew where individuals who are responsible for these activities are, to do as much as I expect the public to do," he said.

Mr Robinson declared: "The Chief Constable has our full support. We will do whatever we can to assist him and as this community moves through these difficult times the Chief Constable will know that it's a united community supporting the full rigour of law and supporting the due process of law."

Sir Hugh said he had no intention of calling for the Army to come back on the streets. He added: "These people have no support from any part of the community in Northern Ireland. They are isolated, they are dangerous, let's be clear on that, they have proved that particular point."

Police are concerned that the deaths of three members of the security forces in three days may spark retaliation from loyalist paramilitary groups, who have been relatively inactive in recent months.

Sir Hugh briefed Mr Robinson and Mr McGuinness on the police response to the crisis and immediately afterwards all three stood outside Stormont and urged everyone in Northern Ireland to back the police in helping to track down the killers.

The Tory leader, David Cameron, described the murders as "shocking," but insisted they would not undermine political progress. "Northern Ireland is not on the brink, it is not staring into the abyss," he said. "It has been transformed over recent years and these people who committed these hideous crimes represent nobody and nothing for the future of Northern Ireland."