The defense began to make its case as the retrial of accused arsonist and murderer Ronald Kemppainen continued Thursday.
The main witness to take the stand was expert fire consultant Dennis Smith.
Though Smith never actually investigated the fire scene at 116 Quincy Street, he took the stand and attempted to disprove the conclusion of three other investigators who'd determined the fire was arson.
He testified Thursday afternoon that it was impossible to determine the fire's cause because several key indicators--a competent heat source, a first fuel and an ignition sequence--had not been found.
As the fire burns for a longer period of time, the cause becomes more difficult to determine, said Smith. It's particularly true in cases with full room involvement.
During cross-examination, the prosecution used quotes from a guide Smith helped write to contradict parts of his testimony.
The trial resumes at 8:30 Friday morning.