(Reuters) — An air safety investigation of a battery fire on a Boeing Co. 787 Dreamliner last month has narrowed the source to one of the battery's eight cells, though the actual cause of the fire is not yet known, the head of the National Transportation Safety Board said on Thursday.

Regulators should now reconsider the assumptions used to certify the plane's lithium-ion batteries, Deborah Hersman said at a news conference in Washington, adding to the uncertainty of when the cutting-edge plane might be cleared to fly again.

The 50 Dreamliners in service were grounded worldwide on Jan. 16, after a series of battery incidents, including a fire on a parked 787 in Boston and an in-flight problem on another plane in Japan. The groundings have cost airlines tens of millions of dollars, with no end in sight.

The NTSB probe is focused on the Boston fire, and Ms. Hersman said investigators now believe multiple short circuits in a single cell may have led to a chemical reaction known as a thermal runaway, which cascaded to other cells and spread the fire.

Ms. Hersman said a review is needed of the "special conditions" under which aviation regulators approved Boeing's use of this particular battery technology on the 787, a decision that has lately come under close scrutiny.

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

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The NTSB plans to issue an interim factual report in 30 days, though the decision on returning the plane to regular flight rests with the Federal Aviation Administration.

As Ms. Hersman was addressing the news conference in Washington, the first 787 flight since mid-January left Texas for Washington state, a ferry flight being run under heavy conditions to see if any battery problems crop up.

According to flight tracking website FlightAware, it left Dallas at 9:25 a.m. CST for the nearly three-and-a-half hour flight to Everett, Wash. Ultimately scheduled for delivery to China Southern Airlines, the aircraft has not yet been handed over to the customer.

The Federal Aviation Administration said Wednesday it had approved the flight, which differed from Boeing's request to run a series of test flights. It placed a number of conditions on the one-off trip, mostly having to do with testing and monitoring the plane's battery.

While the investigation continues, Boeing is pursuing multiple ways to mitigate and contain a fire, if one starts in the batteries, one source familiar with the probe told Reuters. Three or four varying approaches would be pursued to ensure the batteries did not breach their containment systems, even if they caught fire, said the source.