The findings released on Thursday are casting doubt on whether the airliner's troubles can be quickly solved.
The National Transportation Safety Board is investigating last month's battery fire in a Japan Airlines 787 "Dreamliner" while it was parked at Boston's Logan International Airport.
The results so far contradict some of the assumptions made about the battery's safety as part of its government approval, said NTSB chairman Deborah Hersman.
The investigation's findings were released on the same day as the Federal Aviation Administration said it would allow Boeing to conduct test flights on its 787s to collect data about battery performance while the planes are airborne.
It was unclear when those test flights would begin.
The NTSB investigation found the fire was started by multiple short-circuits in one of the battery's eight cells.
That created an uncontrolled chemical reaction known as "thermal runaway" which spread the short-circuiting to the rest of the cells and caused the fire, Ms Hersman said.
The findings do not match up with what Boeing told the FAA when that agency certified the technologically advanced plane for flight, Ms Hersman said.
Boeing said its tests showed a short-circuit would be contained within a single cell, preventing thermal runaway and fire, she said.
"The assumptions used to certify the battery must be reconsidered," Ms Hersman said.
Boeing's tests also showed the batteries were likely to cause smoke in only one in 10 million flight hours, she said.
But the Boston fire was followed nine days later by another smoking battery in an All Nippon Airways plane that had to make an emergency landing.
The 787 fleet has recorded less than 100,000 flight hours, Ms Hersman added.
The plane that caught fire in Boston was delivered to Japan Airlines less than three weeks before its fire and had recorded only 169 flight hours over 22 flights.
The board's findings appear to raise doubts about the thoroughness of FAA's safety certification of the 787's batteries.
The FAA relies to some degree on the expertise of the manufacturer's engineers, especially in the case of a cutting-edge plane like the 787.
There are also questions as to whether Boeing can fix the problems with the addition of a few quick safeguards.
After the fire in Boston, US 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.
FAA officials have ordered the only US carrier with 787s - United Airlines, which owns six - to ground them.
Aviation authorities in other countries swiftly followed suit and a total of 50 planes operated by seven airlines in six countries are grounded.
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