The government should reassess its safety approval of the Boeing 787 lithium ion batteries, the nation's top accident investigator said Thursday.

The National Transportation Safety Board's investigation of last month's battery fire in a Japan Airlines 787 "Dreamliner" while it was parked in Boston shows the fire started with multiple short-circuits in one of the battery's eight cells, said the board's chairman, Deborah Hersman. 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 any short-circuiting was contained within a single cell, preventing thermal runaway and fire, 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 has recorded less than 100,000 flight hours, Hersman noted.

"There have now been two battery events resulting in smoke less than two weeks apart on two different aircraft," she 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."

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 will be able to make a quick fix that returns the planes to the skies.

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 groundings have become a nightmare for the company, which has about 800 Dreamliner orders from airlines around the world.

The 787 is the first airliner to make extensive use of lithium batteries. Aircraft makers view the batteries, which are lighter, recharge faster and can store more energy than other types of batteries of an equivalent size, as an important way to save on fuel costs. 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 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."