Police and Gardai are trying to establish if four men being treated in hospital in Dublin are linked to an attack on two people in County Armagh.
The remains of the house where the attack happened
Up to six men entered their home at Foley Road, Tassagh, in Keady, at about 2100 GMT on Monday, doused them in a flammable liquid and set them alight.
Lisa McClatchey, 21, is in a critical condition and her partner Thomas O'Hare, 33, is serious but stable.
The four men were admitted to hospitals in County Louth with serious burns.
The men, who are aged between 24 and 34, are understood to be the Smith brothers from Clady.
They have since been moved to St James's in Dublin where a spokesperson described them as being "stable but critical".
Both police forces are monitoring the situation and the PSNI said that given the seriousness of the incident police would check hospitals for recent admissions for burn injuries.
They said that was being carried out in consultation with Irish police, but added that no arrests have been made and they are looking for six suspects.
A forensic examination of the scene has been carried out
Detectives investigating the attack have also searched a house in Clady.
It is understood several items were taken away for further examination during the search on Tuesday afternoon.
Earlier, Detective Chief Inspector David McConville, the officer in charge of the investigation, said it was an unusual and vicious crime and appealed for information.
"As part of our investigation, we will be investigating the victims and the background - all those aspects," he said.
Police have not ruled out paramilitary involvement or self-styled vigilantes and they said they were keeping an open mind.
"It has to be said that no one has the right to act as judge and jury, if that was the motive, and certainly we would call on the public to help us identify these people," Mr McConville said.
"There are six vicious men out there who have attacked two vulnerable people in the sanctity of their own home."
SDLP councillor Sharon Haughey, who lives in the area and is a cousin of the Smith brothers, said locals were "shocked that something of this nature has happened in the quiet countryside".
Ulster Unionist assembly member Danny Kennedy said: "What sort of people can sink so low as to set fire to others?"