Richard Walsh, a Massport spokesman, said the jet's auxiliary power unit battery, which is housed in a small compartment known as the aft electrical equipment bay, exploded after crews believed the fire was extinguished.

WSJ: Japan Airlines' Jet Suffers Fire in Boston

Officials had said they were examining the electrical system broadly, but while earlier incidents involved components in the aft electrical equipment bay, located behind the wings, Wednesday's involved a forward bay that until now had been largely free from scrutiny.

WSJ: Boeing Dreamliner Makes Emergency Landing in Japan

One sits underneath the cockpit in a compartment known as the forward electrical equipment bay, 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.

WSJ: All Boeing Dreamliners Are Grounded