Brothers Martin, Neil, Christopher and Stephen Smith were admitted to hospital in Dundalk with serious burns a few hours after Lisa McClatchey (21) and her partner Thomas O'Hare (33) were beaten with hammers and burnt alive at their home outside Keady in November last year.

The Smith brothers were transferred to a specialist unit at St James's Hospital in Dublin where they remained for several months and police were waiting for their conditions to improve before questioning them about the attack.

However, the Belfast Telegraph has learned that after the Co Armagh men signed themselves out of hospital they failed to return to their homes in Northern Ireland and instead remained in the Republic.

It is understood the PSNI is considering launching extradition proceedings in a bid to have the men returned to Northern Ireland for questioning.

A source close to the investigation said: "Police are taking legal advice on how best to make their move and are considering extradition."

The Belfast Telegraph has also learned that one of the brothers remains seriously ill and in need of round-the-clock care.

Their father Colum Smith (57), from Mourneview, Mowham, in Co Armagh, appeared in court in February accused of taking people he knew or believed were guilty of attempted murder out of Northern Ireland by car.

Another man, David Harold Graham of Shergrim Glen, Omagh, Co Tyrone, appeared in court in November on charges of withholding information and perverting the course of justice.

It is alleged that Graham lent a car to one of the brothers.

Up to six men forced their way into the remote bungalow where the couple lived on November 6 last year before hitting Mr O'Hare and Ms McClatchey with hammers, dousing them with petrol and setting them alight.

Ms McClatchey managed to run from the house in flames and alerted neighbours. Mr O'Hare was found by neighbours lying in the garden. He died four days later in hospital.

Ms McClatchey, who was admitted to hospital with 80% burns, died a week later.

After the attack police contacted hospitals in the province and across the border to find out if anyone had been admitted with burn injuries, which is when their attention was drawn to the Smith brothers.

It is understood Mr O'Hare was the main target of the attack. At the time police would not rule out the possibility the attack was carried out by self-styled vigilantes.

A PSNI spokesman said that investigations into the double murder are ongoing. He added: "Police do have suspects and are considering our options on how to advance the investigation."