United Technologies, which builds the aircraft's auxiliary power unit, said it would also cooperate.

BBC: Dreamliner: Japan and US probe battery maker

Flight 186's auxiliary power unit failed during the wait and the aircraft had to return to a gate.

WSJ: Flight Nightmares Overshadow Airlines�� Improving Record

It said it would now widen its probe to look at the battery charger and the auxiliary power unit.

BBC: Dreamliner: Japan and US probe battery maker

Officials found that the battery for the aircraft's auxiliary power unit was incorrectly connected to the main battery that overheated.

BBC: Boeing 'set to offer plan to fix Dreamliner battery'

Donahue says it's not likely this could have happened during flight when the auxiliary power unit was not in use.

CNN: Fire aboard empty 787 Dreamliner prompts investigation

The main investigation, which also involves the FAA and Boeing, focuses on the 787's cutting-edge lithium ion battery system that powers the plane's auxiliary power unit.

CNN: Quick fix for Dreamliner looks less likely, experts say

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

The 787 has two identical lithium-ion batteries, one of which is located toward the front of the plane and powers cockpit electrical systems, the other toward the rear and used to start an auxiliary power unit while the plane is on the ground, among other functions.

NPR: FAA Approves Resumption Of Boeing 787 Flights

The ES is critical to aircraft operation it distributes power around the aircraft from the engines and the Auxiliary Power Unit (APU) which provides the power to operate the air conditioning and to start the engines at the gate to all the systems requiring electricity.

FORBES: Can Boeing Contain the Dreamliner's Technical Glitches?