A KING5 News source says Boeing has narrowed down the cause of two battery failures aboard 787s to four or five causes.

To test those theories, Boeing wants to put one of its test airplanes back in the air. Those theories include things like vibration and the temperature inside the forward and rear electrical bays where the lithium ion batteries are stored.

Meanwhile, the nation's top air safety investigator looking into the burning batteries on Boeing's 787 Dreamliner said the investigation will review the assessments that were made by the Federal Aviation Administration in approving the plane and what more needs to be done before the grounded jets take flight again.

Deborah Hersman, chairman of the NTSB, said Wednesday investigators were probably weeks away from determining the root cause. But whatever cause is found, Hersman says any risks from the lithium ion batteries need to be mitigated.

On Thursday, Hersman will hold a news conference in Washington, D.C. that will explore both the technical and regulatory aspects of the battery fire and what lead to the approval of the lithium ion batteries.

On Wednesday, Hersman told the Wall Street Journal, We are evaluating assessments that have been made [by Boeing and the FAA], whether those assessments were accurate, whether they were complied with and whether more needs to be done, she said. And I think that's important, before this airplane is back in the air, to really understand what the risks are and that they're mitigated effectively.

Meanwhile, the FAA is continuing to evaluate a request from Boeing to perform its own test flight to find the cause of the battery failures.

One ferry flight for a Dreamliner has been approved by the FAA. The plane was built in Everett and will be flown back to Everett from a painting contractor in Ft. Worth, Texas. The plane is for an airline in China.

KING 5's Liza Javier contributed to this report.

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