WASHINGTON - The US National Transportation Safety Board (NTSB) said Thursday that it has identified the origin of the Jan 7 battery fire at a Boeing 787 in Boston, indicating the 787 fleet may not resume flight any time soon.

After an examination of the Japan Airlines (JAL) plane's lithium-ion battery, the NTSB investigators determined that the fire started with short circuiting in one cell and then spread to the other seven cells of the battery.

They ruled out both mechanical impact damage to the battery and external short circuiting, and then narrowed the cause to battery malfunction.

During the 787 certification process, Boeing claimed the likelihood of a smoke emission event from a 787 battery was less than once in every 10 million flight hours. But there have been two critical battery events on the 787 fleet within fewer than 100,000 flight hours.

According to Boeing's assessment, any fire would be contained within a single cell and there would be no thermal runaway. But both occurred in the JAL event.

The independent federal agency concluded that not enough layers of defense and adequate check were built into the design, certification and manufacturing of the battery.

The US government has ordered US carriers to ground their 787s since Jan 16. "The decision to return the fleet to flight will be made by the FAA (Federal Aviation Administration)," said NTSB chairwoman Deborah Hersman, adding that the agency would release an interim report of factual findings within 30 days.

The 787 is the first airliner to use lithium batteries, which can recharge faster and store more energy than other types of batteries of an equivalent size.

Airbus also plans to use lithium batteries to build A350, a 787 competitor expected to be launched next year.