Further testing still has not found the cause of a battery fire aboard a Boeing Co 787 Dreamliner in Boston earlier this month, the National Transportation Safety Board said on Sunday.
In a statement released Sunday, the safety regulator said "no obvious anomalies were found" in its initial investigation of an undamaged battery aboard the plane and that a more detailed examination would follow.
The NTSB's preliminary finding, along with the absence of any reported progress in determining a root cause of the auxiliary power unit's battery fire, underscores that the safety investigation and the grounding of 787s may drag on longer than Boeing had hoped.
Oliver McGee, an aerospace and mechanical engineer who was a deputy assistant secretary of transportation for technology policy under President Bill Clinton and a former consultant to Boeing, described the challenge facing the investigators as a "megascale engineering puzzle."
The Dreamliner's troubles have roiled the airline industry, with safety regulators and experts warning that investigations into the cause of a series of small fires on the plane could take months or even a year.
Regulators grounded the Dreamliner on January 16 after a series of safety incidents, including one on a Japan Airlines Dreamliner in Boston and an All Nippon Airways Dreamliner in Japan. The incident in Japan forced a plane to make an emergency landing.
The grounding has forced hundreds of flight cancellations worldwide, including in the United States, India and South America.
The uncertainty over the Dreamliner has raised questions for Boeing as deliveries for its signature aircraft remain stalled and airlines await the futuristic plane, which was to play a major role in their plans for expansion.