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Detective Chief Inspector Gary Pettiford said he could not comment on what led to the arrest but added three people had come forward to assist with their investigation.
"We don't believe that she was known to either of the deceased, or Rebecca, Todd's fiancée," he said.
"We still have a hell of a lot of work to do, and it's all quite sensitive."
Pettiford confirmed the woman went to school in Biggenden.
Police and SES volunteers have been combing the area for clues.
"We have conducted numerous searches with the SES both at the scene of the fire but also at the Isis Highway over a number of days, we are in fact still searching."
He said police divers were searching a body of water in Ban Ban Springs today.
"We have found some items of interest but, again, I can't comment on the nature of those items," he said.