A 37-year-old man detained by gardai in Co Donegal on Friday on an unrelated matter is understood to be a suspect in the murder.
The PSNI said they are no longer looking for anyone in the region in relation to the killing.
In regard to the man in custody in Donegal, a PSNI spokesman added: "There are a number of options we will now be considering to progress this investigation and seeking an extradition order is one of these options."
Ms Dornan's house in Hazel View in the Lagmore area was so badly damaged by the fire the roof collapsed. Officers believe the killer torched the property in a bid to destroy evidence.
The mother of two boys and a girl, all aged under nine, was stabbed with a 14in kitchen knife.
At her requiem mass on Friday, parish priest Fr Martin Graham condemned the crime.
"Whoever did this to Jennifer has turned her death, which was cold-hearted and callous already, into something that was wicked and obscene," he said.
A 39-year-old man arrested last week on suspicion of withholding information was released pending a report to the Public Prosecution Service. An online campaign to raise funds for Ms Dornan's family has generated over £16,000.