WASHINGTON -- The government should reassess its safety approval of the Boeing 787 lithium-ion batteries, the nation's top accident investigator said Thursday, casting doubt on whether the airliner's troubles can be quickly remedied.The National Transportation Safety Board is investigating last month's battery fire on a Japan Airlines 787 Dreamliner while it was parked in Boston. The results so far contradict some of the assumptions made about the battery's safety at the time the system won government approval, board Chairwoman Deborah Hersman said.The inquiry shows that the fire started with multiple short circuits in one of the battery's eight cells, she said. That created an uncontrolled chemical reaction known as "thermal runaway," which is characterized by progressively higher temperatures. That spread the short circuiting to the other cells and caused the fire, she said.The findings are at odds with what Boeing told the Federal Aviation Administration when that agency was working to certify the company's newest and most technologically advanced plane for flight, Hersman said. Boeing said its testing showed that even when it tried to induce short circuiting, any fire was contained within a single cell, preventing thermal runaway and keeping the fire from spreading, she told reporters at a news conference.Boeing's testing also showed that the batteries were likely to cause smoke in only 1 in 10 million flight hours, she said. But the Boston fire was followed nine days later by a smoking battery on an All Nippon Airways plane that made an emergency landing in Japan. The 787 fleet has recorded fewer than 100,000 flight hours.The plane that caught fire in Boston was delivered to Japan Airlines less than three weeks earlier and had recorded only 169 flight hours over 22 flights."There have now been two battery events resulting in smoke less than two weeks apart on two different aircraft," Hersman said."This investigation has demonstrated that a short circuit in a single cell can propagate to adjacent cells and result in smoke and fire. The assumptions used to certify the battery must be reconsidered."All 787s have been grounded since Jan. 16. But on Thursday, the FAA allowed Boeing to conduct a "ferry flight" from Fort Worth's Meacham Airport, where the 787 was being painted for China Southern Airlines, to Boeing facilities in Everett, Wash. Boeing said the flight was "uneventful," adding that the crew monitored the battery throughout.The problem has become a nightmare for Boeing, which has about 800 orders for the aircraft from airlines around the world. The company's customers were already frustrated that the 787 was more than three years late when the first one was delivered in late 2011.The safety board's findings appear to raise doubts about the thoroughness of the FAA's safety certification of the 787's batteries and whether Boeing can remedy the problems with a few quick safeguards.The FAA typically delegates testing of new aircraft designs to the manufacturer while overseeing that the tests meet the agency's requirements. The agency also relies to some degree on the expertise of the manufacturer's engineers.After the Japan Airlines fire at Logan Airport, Transportation Secretary Ray LaHood and FAA Administrator Michael Huerta ordered a review of the 787's design, certification, manufacture and assembly. That review is still under way.The significance of the safety board's findings "is if this can happen -- and the safety analysis assumed that it would not happen -- then the safety analysis is no longer valid," said John Hansman, an aeronautics professor at the Massachusetts Institute of Technology and a member of the FAA's Research and Development Advisory Committee.