Japanese investigators on Thursday began their probe into the cause of a midair emergency on a Boeing 787 Dreamliner domestic flight as the government ordered the grounding of all 22 aircraft operated by the nation's two main carriers.

Japan aviation safety officials in Takamatsu, southwestern Japan, where the plane made an emergency landing on Wednesday, said the inside of the powerful lithium ion battery that has been connected to malfunctions on the aircraft appeared burned and swollen.

"It was like charcoal," said Hideyo Kosugi, one of the five investigators of the Japan Transportation Safety Board dispatched to the site.