TOKYO--The head of a Japanese transport ministry investigation into battery problems on a Boeing 787 Dreamliner said Tuesday that some cells in a battery taken from the plane were found to be swollen, the latest finding in the slowly progressing probe.

The swollen cells were discovered in the battery for an auxiliary power system used on the All Nippon Airways (9202.TO) operated Dreamliner involved in an emergency landing last month after smoke was detected coming from another battery on the jet.

The investigators found the stainless steel cases of two of the eight cells in the APU battery were swollen, said Masahiro Kudo, head of the Japan Transport Safety Board investigation team.

"We don't know yet if this is any anomaly" or if the swelling is within design specifications, Mr. Kudo said.

The emergency landing of the ANA Dreamliner, along with a battery fire on a 787 at Boston airport, prompted the global grounding of the high-tech jet.

Write to Yoshio Takahashi at yoshio.takahashi@dowjones.com

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