A young child has become the fourth victim of the fatal house fire in Fermanagh, the PSNI said.
Post-mortem examinations will take place to positively identify the victims of Tuesday's early morning blaze.
A man aged 27 is being questioned on suspicion of murder after he was detained at the scene in Derrylin and taken to hospital for treatment.
He has been moved to police custody for questioning.
PSNI detective inspector Peter McKenna said: "It is with great sadness that I can now confirm that a fourth body has been found within the crime scene and that it appears to be the body of a young child."
The whitewashed rural bungalow was destroyed in the blaze and the roof collapsed, leaving a shell surrounded by farm buildings and machinery.
The victims only moved to the area recently, a local priest said, and were members of one family.
They had previously lived in the Republic and were originally from England.
Mr McKenna added: "I believe the fire was started deliberately and we are working hard to progress the investigation into this very complex crime scene.
"There is significant structural damage caused by the ferocious fire and the scene examination is further compounded by the severe weather."