The Rev Ian Paisley, the leader of the Democratic Unionists — who are preparing to say whether they will go into government with Sinn Fein — said that the crime was a “litmus test” for the Provisional IRA’s political wing.

Police said that they wantedto question four men, all brothers, who were critically ill last night in the burns unit of St James’s Hospital, in Dublin. The home of two of the brothers was raided by police investigating the attack, which left Lisa McClatchey, 21, and Thomas O’Hare, 33, in the intensive care unit of the Royal Victoria Hospital, in Belfast.

Ms McClatchey, a Protestant from a loyalist housing estate in Portadown, who is also related to a former senior Orangeman, raised the alarm when she ran in flames from the house that she shared with Mr O’Hare, a Roman Catholic, in an isolated country area near the village of Keady.

It is believed that up to six masked men burst into their home and set it on fire during a ferocious attack. Forensic science experts were examining the blackened wreckage of the house yesterday. Neighbours who carried Mr O’Hare from the inferno said that he was in agony and covered in burns.

Hours after the first report of the attack, four men presented themselves at a hospital across the border in Dundalk. Their burns were so serious that they were transferred to a specialist unit in Dublin.

A Police Service of Northern Ireland spokeswoman said: “Police searched a house in Clady, Co Armagh, yesterday as part of their investigation into the attempted murders.”

Sources close to the inquiry confirmed that police believe that at least two other people know what happened when the gang went into Ms McClatchey’s and Mr O’Hare’s home.

Police in the Republic are waiting to question the four men, who are not under armed guard. It is understood that they are aged between 24 and 34 and are from the South Armagh area.

The attack on the couple drew widespread criticism, with Conor Murphy, the local Sinn Fein MP, the nationalist SDLP and Ulster Unionists all expressing revulsion. Sharon Haughey, a local SDLP councillor, suggested that Mr O’Hare was the target of a vigilante attack and it is understood that he may recently have had a “confrontation” in the area.

Mr Paisley said: “This attack is yet another litmus test for Sinn Fein/IRA and their attitude to the PSNI and the rule of law. They have an inescapable responsibility as leaders in their community to encourage people to go directly to the police with all and any information available to help convict those responsible for these brutal deeds.”

Mr Murphy said yesterday, in reply to a question, that he had “no difficulty” with people going to the police if they wished or passing on information about the attack via other means. He said that the details of the incident were difficult to comment on as “it may soon be sub-judice”. Detectives have ruled out a sectarian or racist motive but are following a definite line of inquiry. They have not dismissed paramilitary involvement.

The DUP has called on Sinn Fein to endorse publicly the PSNI and to encourage its supporters to report crime to the police, one of the DUP’s conditions for entering a power-sharing government with republicans next March.

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