A man and woman have been doused with flammable liquid and set on fire.
The house was badly damaged in the attack
Police said a gang of up to six men entered a house at Foley Road in Tassagh, near Keady in County Armagh, at about 2100 GMT on Monday.
They set the pair and the house alight. Lisa McClatchey, 21, is in a critical condition with serious burns.
Her partner Thomas O'Hare, 33, is serious but stable. Police said those who carried out the attack are "vicious men" and have appealed for information.
Ms McClatchey raised the alarm by running to a neighbour's house.
Police have ruled out a sectarian or racist motive, but are following a
definite line of inquiry and said some sort of paramilitary involvement was a possibility.
Detective Chief Inspector David McConville, the officer in charge of the investigation, said it was an unusual and vicious crime.
"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, 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?"