(Reuters) — Japanese investigators probing a lithium-ion battery meltdown on a Boeing Co. 787 jetliner a year ago are looking at a battery that overheated on a Dreamliner in Tokyo this month to help unlock the cause of the earlier fire, an official from the Japan Transport Safety Board said on Wednesday.

The incident on board an ANA Holdings 787 a year ago left the battery charred and deformed, destroying evidence that would have pointed to a cause. The latest event on a parked Japan Airlines in a redesigned battery packed with insulation destroyed only one of eight cells.

"The remaining seven cells are untouched, and I think that is where the investigation will focus," Masahiro Kudo, the lead investigator on the ANA battery said during a press briefing.

That overheating and one a few days earlier on a 787 parked at Boston's Logan airport prompted aviation regulators in the U.S., Japan and elsewhere to ground the global fleet of Dreamliners for more than three months.

Authorities, without discovering the root cause of the meltdown, allowed Boeing to get its carbon composite back into the air after it redesigned the battery with insulation, a vent to eject any hot gases out of the aircraft, and encased it in a steel box to contain any fire. Finding the reason for the overheating could spur further design changes.

The United States National Transportation Safety Board, which is looking at the incident in Boston, has sent accident investigator Mike Bauer to join the latest probe. The JAL Dreamliner's battery emitted smoke at Tokyo's Narita Airport just before take off. Japan's Civil Aviation Bureau is in charge of the investigation.

In the year since the first overheating, the number of 787s in operation has more than doubled to 115 planes at 16 carriers. ANA is the world's leading operator with 24 of the state-of-the-art jetliners built with carbon-fiber composite materials and a powerful electrical system to reduce weight and improve fuel efficiency.