WASHINGTON -- The results of the National Transportation Safety Board's investigation into the causes of a battery fire on a Boeing 787 Dreamliner are expected in the coming weeks, NTSB chair Deborah Hersman said Wednesday.

"We're probably weeks away from being able to tell people what happened and what needs to be changed," Hersman said at a news conference.

The NTSB chief said investigators were "proceeding with a lot of care" in probing the cause of a Jan. 7 lithium-ion battery fire on a Japan Airlines 787 that occurred as the plane sat on the tarmac at Boston's Logan airport.

Hersman said the most concerning issues seen in the probe so far were short circuits and thermal runaway, an uncontrolled chemical reaction that produces rising temperatures.

"These factors are not what we expected to see in a brand-new battery," she said.

The battery problem on the JAL 787, and another on an All Nippon Airways 787, led to a global grounding of all 50 Dreamliners in service until the issue is fixed.

The NTSB will hold a news conference Thursday to update the public on the 787 investigation, Hersman said.

Copyright Agence France-Presse, 2013