Deborah Hersman, chairwoman of the U.S. National Transportation Safety Board, said the investigation will look at "the assessments that were made" initially by Federal Aviation Administration officials in approving the plane, The Wall Street Journal reported.
Hersman also told reporters investigators were "probably weeks away from being able to tell people, 'Here is what exactly happened and what needs to change.' "
The FAA grounded U.S.-registered Dreamliners Jan. 16 following a fire in the lithium-ion battery on a parked Japan Airlines Co. in Boston Jan. 7 and a burning battery on an All Nippon Airways Co. Dreamliner forced to make an emergency landing in Japan a week later.
Global regulators followed the FAA example, grounding all 50 Dreamliners operated by eight airlines worldwide.
Hersman said the board is not only looking at the cause of the battery failures but how well the FAA assessed the potential hazards of Boeing's lithium-ion installations years earlier, the Journal said.
The Journal said neither U.S. nor Japanese investigators had identified a root cause for the battery problems.