This article was reproduced with the kind permission
of the British Broadcasting Corporation
19 November 2007, 10:59 GMT
Mr McElhill, a registered sex offender, is suspected of starting the blaze at the house at Lammy Cresent in Omagh.
The Western Health and Social Care Trust has said it has never placed a child with the McElhills.
It is unclear as to whether or not the girl was a family friend or had been placed there by another agency.
However, police became concerned after learning that she had been staying in the house.
Mr McElhill was first convicted of indecently assaulting a 17-year-old girl in March 1994.
He received a two-year jail term, which was suspended for three years.
Before the suspended sentence had expired, he sexually assaulted another 17-year-old in September 1996, for which he was jailed for three years in April 1998.
He walked free in August 1999 having already served 18 months while on remand.
Police said they would not comment as their murder inquiry relating to the fire was continuing.
All seven deaths are being treated by detectives as murder.
Police said petrol had been scattered around the house and it had been set alight.
The family died of smoke inhalation.
Mr McElhill, 39, his partner Lorraine McGovern, 30, and their five children, Caroline, 13, Sean, 7, Bellina, 4, Clodagh, 19 months and James, who was nine months old, were all killed in the blaze last Tuesday.