U.S. safety investigators have ruled out excess voltage as the cause of a battery fire this month on a Boeing Co. 787 Dreamliner jet operated by Japan Airlines Co. Ltd. and said they were expanding the probe to look at the battery’s charger and the jet’s auxiliary power unit, Reuters reported.

Last week, governments around the world grounded the Dreamliner while Boeing halted deliveries after a problem with a lithium-ion battery on a second 787 plane, flown by All Nippon Airways Co. Ltd., forced the aircraft to make an emergency landing in western Japan.

A growing number of investigators and Boeing executives are working around the clock to determine what caused the two incidents which the U.S. Federal Aviation Administration says released flammable chemicals and could have sparked a fire in the plane’s electrical compartment.

There are still no clear answers about the root cause of the battery failures, but the U.S. National Transportation Safety Board’s statement eliminated one possible answer that had been raised by Japanese investigators.

It also underscored the complexity of investigating a battery system that includes manufacturers around the world, and may point to a design problem with the battery that could take longer to fix than swapping out a faulty batch of batteries. Read more.