The National Transportation Safety Board said Monday it is examining whether the Boeing 787's lithium-ion battery problems were due to formation of microscopic structures called dendrites, which can cause overheating.
The battery problems caused the entire Boeing 787 Dreamliner's fleet to be grounded last month.
Dendrites are tiny deposits of lithium that can grow inside a battery's cells. They can cause short circuits, heat or even fire.
On January 16, the 50 Dreamliners in service around the world were grounded after a battery fire on a Japan Airlines (JAL) plane parked in Boston, and battery smoke on an All Nippon Airways flight forced an emergency landing.
"As part of our continuing investigation, we are certainly looking at whether dendrites may or may not have been a factor" in the faulty batteries, said NTSB spokewoman Kelly Nantel.
Nantel did not say if dendrites had in fact been found, and stressed that investigators were still considering several potential causes for short circuiting that was observed.
"They include looking at the state of charge of each individual cell and the method and delivery of that charge, looking for evidence of contamination, electrode folds, wrinkles and pinches and the assembly of the cells and battery and looking at the total design of the battery, including the physical separation of the cells, their electrical interconnections, and their thermal isolation from each other," Nantel said in an email to AFP.
The NTSB, which is probing the Boston incident in particular, said last week it had focused on how the fire spread in the battery, rather than on the cause of it.
Boeing declined comment Monday.
Boeing conducted a flight test of a 787 Monday to see how the batteries behaved.