TOKYO (AP) – Japanese and U.S. investigators began a probe into the maker of the lithium ion batteries used in Boeing’s grounded 787 jets.

Tsutomu Nishijima, a spokesman for GS Yuasa, the battery manufacturer, said investigators visited the company’s headquarters in Kyoto, Japan, on Monday and that Yuasa was cooperating with the probe.

All 50 of the 787 Dreamliners that Boeing has delivered to airlines were grounded after an overheated battery forced the emergency landing of an All Nippon Airways 787 flight last week in western Japan. Boeing has halted deliveries of new planes until it can address the electrical problems.