Those tests apparently led the airplane builder to greatly underestimate the chances of battery failure, the safety board said.

CNN: Boeing can conduct Dreamliner test flights

As part of their investigation, Japanese and U.S. experts want to determine if an internal battery failure, or defective lithium-ion cell, was the ignition source in either of the two incidents.

WSJ: All Boeing Dreamliners Are Grounded

"The safety of the flying public is our top priority and we won't allow the 787 to return to commercial service until we're confident that any proposed solution has addressed the battery failure risks, " it said.

BBC: Dreamliner: Japan's ANA cancels more Boeing 787 flights

"Working with internal and external experts in battery technology, we have proposed a comprehensive set of solutions designed to significantly minimize the potential for battery failure while ensuring that no battery event affects the continued safe operation of the airplane, " said Conner.

CNN: FAA approves proposal for fixing Dreamliner battery woes

What changed between Jan. 11 and FAA's issuance of a grounding order on Jan. 16 was that a second battery failure occurred on an All Nippon Airways 787 while the airliner was in flight, said Huerta, who joined LaHood at the luncheon.

NPR: US Officials Defend Handling Of Boeing 787 Mishaps