Japan’s Civil Aviation Bureau is sending investigators looking into problems with Boeing 787 batteries to Seattle, where the aircraft are assembled.

The Transport Ministry said members of the team working on the investigation would leave Tokyo on Sunday for Seattle. It provided no further details.

The U.S. Federal Aviation Administration and Boeing earlier dispatched investigators to join the probe in Japan.

All 50 Boeing 787s in use were grounded after a lithium-ion battery in a 787 flight by All Nippon Airways on Jan. 16 overheated, forcing an emergency landing. Earlier in January, a 787 operated by ANA’s rival Japan Airlines suffered a battery fire while parked at a Boston airport.

Investigators on both sides are trying to determine why the batteries have overheated and how to fix the problem.