Just what the airline industry needs in the dead of winter. U.S. regulators inspecting the now grounded Boeing 787 Dreamliner aircraft said Thursday that they would need more time—stoking fears that aircraft’s grounding could be even longer. While the investigators have found “symptoms” of what caused the Jan. 7 battery fire in Boston, the underlying cause still remains a mystery, said Deborah Hersman, the chairman of the National Transportation Safety Board. Calling the Dreamliner mishaps an “unprecedented event,” Hersman refused to answer questions about how long the investigation could take. The 787 has been grounded worldwide since two fires aboard aircraft in Japan and the incident in Boston, all of which occurred earlier this month.