It is indeed interesting that there are reports of fire having started in the battery compartment or the avionics bay. I found these quotes from Boston Herald interesting:
“We don’t want to speculate, but it could have happened after it arrived,” said Freni, adding that all luggage was off the plane at the time the fire was reported.
Massport Fire Chief Robert Donahue said firefighters saw “heavy smoke conditions” in the entire cabin when they arrived and determined the source of the smoke was the plane’s avionics section where the batteries are located.
(From
Boston Herald)