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 in a Japan Airlines 787 “Dreamliner” while it was parked in Boston. The results so far contradict some of the assumptions that were made about the battery’s safety at the time the system won government approval, said the board’s chairman, Deborah Hersman.

The investigation shows 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 hotter temperatures. That spread the short-circuiting to the rest of the 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 trying to induce short-circuiting, the condition and any fire were contained within a single cell, preventing thermal runaway and fire from spreading, she told reporters at a news conference.

Boeing’s testing also showed 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 in an All Nippon Airways plane that made an emergency landing in Japan. The 787 fleet has recorded less than 100,000 flight hours, Hersman noted.

The plane that caught fire in Boston was delivered to Japan Airlines less than three weeks before the fire 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. With no end in sight, the halt has become a nightmare for Boeing, which has about 800 orders for the craft 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 toward the end of 2011.

Boeing loses money on each 787 it delivers, and the cash burn grows with each missed delivery, analysts have said.

Investigators are still trying to determine why the first battery cell short-circuited, but the board’s findings appear to raise doubts about the thoroughness of FAA’s safety certification of the 787’s batteries and whether Boeing can remedy the  problems with the addition of 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, especially in the case of a cutting-edge plane like the 787.

Following the Japan Airlines fire at Logan International 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 underway.

“We must finish this work before reaching conclusions about what changes or improvements the FAA should make going forward,” LaHood and Huerta said in a joint statement Thursday. “The leading experts in this field are working to understand what happened and how we can safely get these aircraft back into service.”

But John Goglia, a former NTSB board member and aviation safety expert, said NTSB’s findings mean the government will likely require Boeing to re-certify the batteries.

“Certifications aren’t exactly painless and quick,” he said. “It could be a big, drawn-out thing.”

The significance of the NTSB’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 Jon Hansman, a Massachusetts Institute of Technology aeronautics professor and a member of the FAA’s Research and Development Advisory Committee.  

Battery experts said Boeing could try to build more safeguards into the battery by using a greater number of smaller cells and putting more insulation between them. Or, they said, the aircraft maker could switch to a different type of lithium ion battery already approved for aviation. Some business jets use lithium ion batteries as their main batteries.

Switching to another type of battery, such as lead-acid or nickel-cadmium battery, is another possibility, but that would involve changing the charging system as well, they said, and add weight. The new batteries — and, presumably, a revised charging system — would need to be designed and tested by Boeing and approved by the FAA before they could be installed.

The same day as the ANA emergency landing, FAA officials ordered the only U.S. carrier with 787s — United Airlines, which has six of the planes — to ground them. Aviation authorities in other countries swiftly followed suit. In all, 50 planes operated by seven airlines in six countries are grounded.

The 787 is the first airliner to make extensive use of lithium batteries. Besides being lighter, the batteries recharge faster and can store more energy than other types of batteries of an equivalent size, and can be molded to fit into odd spaces on planes. The Airbus A350, expected to be ready next year, will also make extensive use of lithium ion batteries. Manufacturers are also looking to retrofit existing planes, replacing other types of batteries with lithium ion.

But lithium batteries in general are more likely to short-circuit and start a fire than other batteries if they are damaged, if there is a manufacturing flaw or if they are exposed to excessive heat.

In 2007, the FAA issued special conditions that Boeing had to meet in order to use lithium ion batteries in the 787, because at that time the agency’s safety regulations didn’t include standards for such battery systems.

The 787 relies to a greater extent than any previous airliner on electrical systems, as opposed to hydraulic or mechanical ones. The batteries help run those electrical systems and also are used to start a power-generating engine in the rear of the aircraft.

The batteries are made by GS Yuasa of Japan. Japanese aviation investigators probing the cause of the ANA battery failure have also found there was thermal runaway.

Investigators have ruled out mechanical damage or external short-circuiting as possible causes of the initial, internal battery short-circuiting, Hersman said. Investigators and technical experts are now looking for evidence of flaws inside the batteries like pinches, wrinkles or folds, she said.

“We are looking at a number of scenarios,” Hersman said, including the state of charge of the battery, its manufacturing processes and the design of the batteries.

“We haven’t reached any conclusions at this point,” she said. “We really have a lot of work to do.”