Federal investigations said Sunday that they had ruled out excessive voltage as the cause of a battery fire on a Boeing 787 in Boston this month, widening the mystery into what led to the grounding of the world’s most technologically advanced jet after a second battery-related problem last week.

With investigators focused on the plane’s lithium-ion batteries, the National Transportation Safety Board said an examination of the data from the plane’s flight recorder indicated that the battery “did not exceed the designed voltage of 32 volts.” The fire aboard a Japan Airlines plane on Jan. 7 at Logan International Airport in Boston occurred after the passengers had gotten off.

Last week, a battery problem on another 787 forced an All Nippon Airways jetliner to make an emergency landing in Japan. That episode prompted aviation authorities around the world to ground the plane, also known as the Dreamliner. The Federal Aviation Administration said last week that it would not lift the ban until Boeing could show that the batteries were safe.

The safety board did not address the grounding issue or provide a timetable for its investigation, which industry experts said could take months.