US government investigators said Thursday that they were very concerned over what they called an \"unprecedented\" battery fire on a Boeing 787 Dreamliner. The fire, and a second 787 battery problem which forced an emergency landing of an aircraft in Japan, forced aviation authorities in the US and elsewhere to order the global grounding of all 50 787s in operation worldwide. \"This is an unprecedented event. We are very concerned. We do not expect to see fire events on board aircraft,\" said Deborah Hersman, chairwoman of the National Transportation Safety Board. Hersman said the NTSB had still not reached any conclusions on what caused the January 7 battery fire aboard the empty JAL 787 plane parked at Logan airport in Boston. But she said that testing so far showed that the battery experienced a thermal runaway, short circuits and a fire. But she said that the board was still far from reaching any conclusion on what caused the fire. \"We are early in our investigation,\" she said. \"We are working very hard to determine what happened and why it happened.\"