A 43-year-old man is being questioned in connection with the murder of Thomas O'Hare and attempted murder of his partner, Lisa McClatchey, in Co Armagh.
The PSNI launched a murder investigation following the death of Mr O'Hare, who was set alight by intruders in his home on Monday.
The 33-year-old died last night at a hospital in Belfast.
Ms McClatchey, who was also attacked in the incident, remains in a critical condition in hospital.
The couple suffered horrific injuries when they were doused in flammable liquid and set alight by a gang of masked men at the home they shared outside Keady.
Just hours after the blaze four brothers, aged between 24 and 34, presented themselves at the accident and emergency unit of Our Lady of Lourdes Hospital in Drogheda, Co Louth.
All had suffered burns and have since been transferred to a specialist burns unit at St James's Hospital in Dublin where they remain very seriously ill.
Police on both sides of the border have been working to establish whether they were involved in the attack.
The PSNI believes up to six people were involved in the burning.
The motive for the attack has not yet been confirmed, but it is believed Mr O'Hare was the main target.
