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A mechanic then saw and smelled smoke there before seeing two distinct flames about three inches long at the front of the case holding the plane's lithium-ion battery in the electronics bay. Other managers reported "intense" smoke and "caustic smelling" in the nearly empty passenger cabin before summoning firefighters, who found "a white glow with radiant heat waves" coming from the battery, the report said. The battery was also hissing loudly and leaking liquids and seemed to be reigniting. Standard fire suppressants had little effect, the report said, and a fire captain's neck was burned, he said, when the battery "exploded."

The new details about the fire were included in a preliminary report that indicates the board has still not made much progress in figuring out why a battery in the new Boeing 787 jet parked at the airport burst into flame on Jan. 7.

The report echoed statements by Deborah Hersman, the board's chairwoman, who told reporters last month that the problems seemed to have originated in the battery, when one of the eight cells had a short circuit and the fire spread to the rest of the cells.

While the safety board plans to continue its investigation, it said it would also hold a hearing on the hazards of the new lithium-ion batteries next month.

The 48-page report and nearly 500 pages of supporting materials added a few new technical details about the condition of the charred battery, even if they did not advance efforts to pinpoint the cause of the fire.

The report said the airplane involved in this incident was delivered to Japan Airlines on Dec. 20. At the time of the fire, it had logged only 169 flight hours and 22 flight cycles.

The airline had flown the jet from Narita, Japan, and it touched down in Boston at 10 a.m.

The flight data recorder showed that at 10:04 a.m., the pilots started the auxiliary power unit, which is energized by one of the plane's two batteries, to provide power on the ground. All the passengers and crew members had gotten off the plane by 10:20, and the first reports of smoke came shortly after that.

The recorder showed that the battery failed at 10:21 a.m., and the plane's electronic system automatically shut down the power unit 12 seconds later.

The incident marked the first sign of trouble with the volatile new batteries on the 787. And the planes were grounded worldwide nine days later after another 787 made an emergency landing in Japan when the pilots smelled smoke.

Over the last two weeks, Boeing has told the government that it had identified the most likely ways in which the batteries could fail, and it proposed several fixes. They contend that the changes would minimize the odds of future incidents and protect the plane and its passengers if a problem does arise.The report from the safety board came a day after federal officials said that the Federal Aviation Administration was close to approving tests of Boeing's approach to fixing the batteries on its 787 jets, and the tests could begin next week.

The F.A.A. could still demand changes in Boeing's proposed new battery design if problems develop in the laboratory and flight tests, which will take several weeks. But the decision to start the tests will be a major step in Boeing's efforts to get the jets, which have been grounded since mid-January, back in the air.

The federal approvals are expected late this week or early next week, even though some battery specialists remain concerned that investigators have not found the precise cause of two incidents in which the jetliner's new lithium-ion batteries emitted smoke or fire.

The plan is still subject to approval by Michael P. Huerta, the head of the F.A.A., and Ray LaHood, the transportation secretary, who will be briefed on it over the next several days.

Mr. LaHood said in January that the planes "won't fly until we're 1,000 percent sure they are safe to fly." Department officials said Mr. LaHood and Mr. Huerta had been kept informed of the details of the proposal as it was created, and they are expected to sign off on it.

The safety report restated that it found no signs of electrical short-circuits from outside the battery, and it described in detail the testing it had performed in the last two months both on the battery and on its various external components.

The two-month investigation found nothing unusual in any of the electrical components linked to the battery, which were all tested. These include the battery charger, its monitoring unit, a starter for the auxiliary power unit and other electronics.

 The report detailed a sequence of events lasting about 36 seconds during which the battery performed erratically and drained of its power, and the auxiliary power unit stopped, but did not provide an explanationd.

The battery first showed a one-volt drop in its designated voltage of 32 volts. Three seconds later, the battery current sharply increased for four seconds, indicating that current was flowing into the battery, instead of out of it. The flow of current then began to flow out of the battery but the voltage kept dropping after that. The voltage decreased to zero and jumped back to 28 volts three times.

Under Boeing's plan to redesign the battery, it would add insulation among the eight cells in the battery to minimize the risk of a short-circuit cascading through most or all of them. The company also proposed adding systems to monitor the temperature and activity in each cell. It would enclose the batteries in sturdier steel boxes to contain any fire, and it would create tubes to vent hazardous gases outside the plane.

The 787 is the first commercial airplane to use large lithium-ion batteries for major flight functions.

Boeing has delivered 50 787s to eight airlines, and officials said it could install new batteries in them quickly once a new design was approved. The company has much at stake with the plane, which is the first commercial jet to be built mostly out of lightweight composite materials. Boeing has orders for 800 more of the planes.

Aviation analysts said the plan would probably protect against the main problem that the safety board has identified, a short-circuit in one of the cells that can set off a chemical reaction that leads the battery to overheat.

But investigators in Japan have suggested that something else may have caused the battery on an All Nippon Airways 787 to emit smoke on a flight on Jan. 16. They said the battery may have been hit by a surge of electrical current from another part of the plane.

Donald R. Sadoway, a professor of materials chemistry at the Massachusetts Institute of Technology, said the Japanese data suggested that temperatures might have shot much higher in that battery than in the one on the plane in Boston. If that is true, he said, Boeing and the F.A.A. might need to add more steps to the safety plan to guard against such possibilities.

The safety board is also looking into how the F.A.A. certified the batteries as safe in 2007 when Boeing's design and testing and its adherence to required special precautions were clearly deficient.

Ms. Hersman, the safety board chairwoman, said last month that Boeing's original tests showed no indication the batteries could erupt in flame and concluded that they were likely to emit smoke less than once in every 10 million flight hours.

Once the planes were placed in service, though, the batteries overheated and emitted smoke twice, and caused one fire, after about 50,000 hours of commercial flights.

Raymond L. Conner, the president of Boeing's commercial airplane division, said this week that industry and academic researchers had learned much since then about the volatile batteries. Other company officials said Boeing would also incorporate what it learned from the two recent incidents into its new tests.