That's the word on the cause of the fire that scorched Saint Mary's Church in Amarillo, where federal and local fire officials have concluded their on scene investigation.
We're told the mezzanine is where Monday's flames started.
But investigators don't know whether an electrical outlet, combustable heating source, or something else is to blame.
ATF Agent Jeffrey Fulton says, even though they know where the fire started, the cause could remain undertermined indefinitely. "We know right where the fire started we just can't eliminate every possible cause of every possible accident or ignition source. "
Investigators say so far they found no trace of arson.
While ATF agents have teamed up with the Amarillo Fire Deptartment to investigate the case so far, now the local fire deptartment will now take control of the investigation.
Officials there tell us after interviewing about 50 people in connection with the fire, they still want to conduct more interviews,as many questions remain.