It may take "weeks" to complete an investigation into battery problems that hit two Boeing 787 Dreamliners, according to the US National Transportation Safety Board.
A battery on a Japan Airlines 787 caught fire, while a malfunction forced an All Nippon Airways Dreamliner to make an emergency landing last month.
The incidents led to the grounding all 50 of the 787s in use. The NTSB said that the battery being used in the 787s may not necessarily be unsafe.
NTSB head Deborah Hersman said: "I would not want to categorically say that these batteries are not safe.
"Any new technology, any new design, there are going to be some inherent risks. The important thing is to mitigate them."
She added that the NTSB was "running through the macro level to the microscopic level on this battery". "But I think we are probably weeks away from being able to tell people here's what exactly happened and what needs to change."
A Boeing spokesman told the BBC the firm was "choosing not to comment on Ms Hersman's remarks as the matter was under active investigation".
Meanwhile, Qantas chief executive Alan Joyce said problems with the 787 had done nothing to alter the airline’s orders and options for 50 Dreamliners.
He told broadcaster ABC in Australia: "We believe that Boeing are a great airline manufacturing company, they're a great engineering company and they will fix this problem eventually.
"They're still producing the aircraft, so the production line hasn't stopped. They have stopped delivering aircraft to customers.
"Our aircraft are due to arrive, the first one in August. We haven't been advised of any delay at this stage."
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