Investigators have yet to pinpoint the cause of a battery fire on a Boeing 787 Dreamliner sitting on a runway in Boston, the head of the U.S. National Transportation Safety Board said Thursday.

"We are now working to identify the cause of the short circuit on cell six," Deborah Hersman told a press conference in Washington. "We have not reached any conclusions at this point."

The fire in Boston and other incidents prompted the Jan. 16 grounding of all 50 Dreamliners in service worldwide.

Hersman said the NTSB probe was far from finished.

Almost a month after the Federal Aviation Administration decided to ground Boeing's new jets, officials are remaining mum about the causes of the fire and no date has been set for getting the Dreamliners back into the air.

The certification performed by Boeing determined that the probability of a fire in the Dreamliner's batteries was less than 1 per 10 million flight hours, yet two such events occurred in less than 100,000 flight hours, Hersman noted.

"The assumptions used to certify the battery must be reconsidered," the NTSB director said.

U.S. regulators are working jointly with those of Japan and France to determine the causes of the fires.

Meanwhile, on Thursday the FAA authorized a Dreamliner crew-only test flight between Dallas and Everett, Washington. EFE