Richard Walsh, a Massport spokesman, said the jet's auxiliary power unit battery, which is housed in a small compartment known as the aft electricalequipmentbay, exploded after crews believed the fire was extinguished.
Officials had said they were examining the electrical system broadly, but while earlier incidents involved components in the aft electricalequipmentbay, located behind the wings, Wednesday's involved a forward bay that until now had been largely free from scrutiny.
One sits underneath the cockpit in a compartment known as the forward electricalequipmentbay, which was where Wednesday's malfunction occurred, and a second in the aft bay under the wings, which was the site of last week's fire in Boston.