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The nation's top transportation safety official said Thursday that the Federal Aviation Administration accepted test results from Boeing in 2007 that failed to properly assess the risks of smoke or fire leaking from the batteries on Boeing's new 787 jets.  

Deborah Hersman, the chairwoman of the National Transportation Safety Board, told reporters that Boeing's tests had predicted that the batteries on its new 787 planes were likely to emit smoke less than once in every 10 million flight hours — and showed no indication that the batteries could erupt in flames.

 But once the planes were placed in service, she said, the batteries overheated and smoked twice last month, and caused one fire, after fewer than 100,000 hours of commercial flights.

"The assumptions used to certify the batteries must be reconsidered," Ms. Hersman said.

She said Boeing's tests before the battery was certified, which the F.A.A. oversaw, found no evidence that a short circuit in one of the battery's eight cells could spread to other cells.

But Ms. Hersman said the fire on a 787 parked at an airport in Boston on Jan. 7 started with a short circuit in one cell and then spread to the other cells.

She said investigators have still not been able to tell what caused the short-circuit in that cell and led to a "thermal runaway," overheating up to 500 degrees, that then cascaded to the rest of the cells.

"This investigation has demonstrated that a short-circuit in a single cell can propagate to adjacent cells and result in a fire," she said.

In searching for the cause of the fire on the plane in Boston, Ms. Hersman said the safety board was still looking at the battery's charging mechanism, potential manufacturing defects or contamination, and whether the cells were not as isolated as they should have been. .

Investigators have so far ruled out two possible reasons for the short-circuit — a mechanical or electrical shock from outside the battery.

"We have left a lot of issues on the table," she said. "We have not yet identified what the cause of the short-circuit is. We are looking at the design of the battery, at the manufacturing, and we are also looking at the cell charging. There are a lot of things we are still looking at."

Ms. Hersman said the plane's flight data showed that the battery's voltage unexpectedly dropped from a full charge of 32 volts to 28 volts, which also suggested that problems began in one of the four-volt cells.

Ms. Hersman said it was still too soon to determine whether the battery's casing had performed its job. While the container had sustained fire damage, it is still being evaluated to determine how protective it was. 

The 787 is the first commercial airplane to use large lithium-ion batteries for major flight functions. All 50 of Boeing's 787s that were delivered to airlines have been grounded since mid-January, when a 787 made an emergency landing in Japan after the pilots smelled smoke in the cockpit.

The F.A.A.'s decision to certify the batteries has come under scrutiny in recent weeks. Because airplane regulations did not cover lithium-ion batteries, the F.A.A. approved Boeing's use of the novel technology under nine special conditions that covered the need to contain or vent any hazardous materials.

Ms. Hersman said that because the problems on the Boeing planes were unexpected, the safety board has been trying to understand "the special conditions related to the failure and the outcome we saw — fire and smoke."

And while Boeing calculated the odds of a problem as minuscule, she said, "there have now been two battery events resulting in smoke less than two weeks apart on two different aircraft."

She said the safety board said it would issue an interim report in the next 30 days. Its findings so far suggest that Boeing will have a hard time convincing regulators that it can fix the problems quickly and get the planes back in the air.

Ms. Hersman stressed it would be up to the F.A.A. to decide whether and when to lift the grounding order on the 787. "About two million people travel on U.S. airlines safely every day," she said. "The aviation community has achieved this remarkable record in large part through redundancy and layers of defense. Our task now is to see if appropriate levels of defense and checks were built in the design, certification and manufacturing process."

Boeing picked the new lithium-ion technology because it provided more power than traditional batteries of the same size. Ms. Hersman's comments came a day after the F.A.A. approved one flight of a Boeing 787, with a flight crew but no passengers.

The flight, which took off on Thursday morning, is the first for a 787 since aviation authorities grounded the innovative aircraft last month after two incidents with its lithium-ion batteries. The F.A.A. said it would let Boeing return the 787 from a painting plant in Fort Worth to its plant near Seattle. It has not yet approved flights to conduct tests on the batteries.