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Deborah A.P. Hersman, chairwoman of the board, said a battery that caught fire in a 787 parked at a gate at Logan Airport in Boston showed signs of short-circuiting and of a "thermal runaway," in which a chemical reaction begins to overheat the battery and speeds up as the temperature increases. But Ms. Hersman described these as "symptoms," and pointedly declined to say whether those problems were the cause of the incident, which, combined with a similar event in Japan, has led to the grounding of all 50 of the planes in service.

     "These are all symptoms that something's  wrong," she said. "Understanding what came first and what triggered the next thing, that's information we are working to identify."

     While there were no deaths or injuries, she said, "These events should not happen."

"There are multiple systems to prevent against a battery event like this," she said. "Those systems did not work as intended. We need to understand why."

Ms. Hersman highlighted the seriousness of the problems more bluntly than other officials have, and her comments made clear that Boeing will not be able to get its planes back in the air anytime soon.

The battery damage was so significant, she said, that investigators are having difficulty retrieving information from the battery control system.

Japanese aviation authorities are leading the investigation into the second battery incident, which occurred earlier this month on a 787 flown by All Nippon Airways. It made an emergency landing in Japan after its pilot reported a burning smell in the cockpit while smoke alarms rang.

Ms. Hersman's briefing, the first by the safety board on the batteries, fleshed out some of the questions facing the forensic engineers but did not identify any cause as particularly likely, or rule any out. And some of the tests on the design of the battery take a week to conduct, she said.

With 50 airplanes grounded and Boeing's marquee new product in limbo, engineers working for Boeing, its suppliers and government agencies in this country and in Japan are scrambling to determine what happened. But Ms. Hersman was not making any promises.

"It is really very hard to tell at this point how long the investigation will take," she said. "What I can tell you is we have all hands on deck. We are working hard to determine what the failure mode here is and what actions have to be taken."

"Lithium-ion" is a vague term that is used in the battery industry to describe a variety of chemistries. This particular battery was built specifically for the 787 and, according to the safety board on Thursday, used an aluminum strip coated in lithium cobalt oxide in its positive electrode. That is an older technology and is more prone to thermal runaway; it also generates oxygen as it heats, making combustion more likely.