This article was reproduced with the kind permission
of the British Broadcasting Corporation
14 November 2007, 14:59 GMT
Arthur McElhill, Lorraine McGovern and their five children are thought to have been killed in Tuesday's blaze.
Detective Chief Superintendent Norman Baxter said the inquiry was at an early stage and appealed for information.
"This is one of the most tragic and devastating murder inquiries the PSNI has to date had to encounter," he said.
He said a significant quantity of an accelerant, petrol, had been found in the house.
Five bodies have been found in the house at Lammy Crescent. The search for the remaining two members of the family is ongoing.
The bodies have not yet been taken from the house.
A senior fire investigator is being flown in from England to assist with the investigation.
Earlier on Wednesday, Fire Service Western Area Commander Eoin Doyle said their priority was finding the two people "not yet accounted for".
He said that additional firefighters were being drafted in to help find the bodies and establish how the fire began, just before 0500 GMT on Tuesday.
"We've had the ceilings fall down on top of the belongings and a partial roof collapse, it's going to make it a very slow process for us," he said.
Mr Doyle said that it had been a "harrowing experience" for the local firefighters and trauma support for the crews had begun.
Father Tom McManus, the parish priest in Corlough in County Cavan where Lorraine McGovern grew up, said they had been a very close family.
"It's just devastating when you look back at those things, the happy times and the good times," he said.
"They were reminiscing about that the last time we had spoken together - the last time they had visited, at the baptism... it's just horrendous."
The eldest of the five children, Caroline, 13, attended the nearby Sacred Heart College.
The other four were Sean, 7, Bellina, 4, Clodagh ,19 months and James, who was nine months old.
Sean and Bellina were pupils at St Conor's Primary School, which was closed following the tragedy.