Boeing is confirming that a fire on one of its new 787s appears to have started in a battery, as scrutiny of the problem increases.

Also Tuesday, the National Transportation Safety Board said it will send two more investigators to Boston to examine the Japan Airlines plane. The NTSB says the battery had "severe fire damage."

The fire happened on the ground Monday, with no passengers on board. But in-flight fires can be catastrophic, so the matter is getting close scrutiny by aviation authorities.

United Airlines says it checked its own 787s overnight. It would not say what the inspections found.

Boeing says the problem appears to be unrelated to previous electrical problems on the 787.

Boeing Co. shares have fallen nearly 5 percent since the fire was reported.