A source told the WSJ an improperly installed bundle of wires was found in the aft electrical-equipment bay, under the Dreamliner's cabin.
Those wires are related to the auxiliary power unit battery.
An initial report from the National Transportation Safety Board on the cause of yesterday's fire noted that same unit in the Japan Air Lines Dreamliner "had severe fire damage," indicating it was near the source of the fire, if not the source itself.
This morning, a different Japan Air Lines Boeing 787 at Logan was grounded as it prepared to take off for Tokyo, due to a 40-gallon fuel leak.
Update: A spokesperson for United verified the airline did inspect its six Dreamliners in response to the fire on the Japan Air Lines plane, but could not confirm any results of the inspections.
"We continue to work closely with Boeing" to ensure the reliability of the aircraft, she said.