WASHINGTON -- Firefighters and mechanics tried repeatedly to put out a battery fire aboard a Boeing 787 Dreamliner through smoke so thick that they couldn't see the battery, according to documents released Thursday that portray the incident as more serious than previously described.The Jan. 7 fire at Boston's Logan Airport is under investigation by the National Transportation Safety Board, which released laboratory analyses, interviews and other data it has gathered.It still hasn't pinpointed the cause.Federal Aviation Administration officials are expected to decide within days whether to approve a plan by Boeing to revamp the 787's lithium-ion batteries to prevent or contain fires. Once the plan is approved, Boeing hopes to swiftly test the reconfigured batteries and get the planes back in the air.Dreamliners worldwide have been grounded since a second battery incident led to an emergency landing in Japan nine days after the Boston fire. The incidents have raised questions about the safety of using lithium-ion batteries, which are more susceptible than other batteries to igniting if they short-circuit or overheat. The episodes have also called into question the FAA's process for certifying new aircraft.The safety board said Thursday that it plans two public hearings next month, one to explore lithium-ion battery technology in general and another to discuss the design and certification of the Boeing 787 battery system.The Boston fire occurred aboard a Japan Airlines plane that had just landed after an overseas flight and was parked. The first firefighter to enter the plane reported seeing "a white glow about the size of a softball" through the smoke using his hand-held heat-imaging camera. He applied a fire-extinguishing agent, which reduced the glow.Another firefighter entering the electronics bay reported "no visibility" because of the smoke and directed another burst from a fire extinguisher at a hot spot, but the battery seemed to rekindle. This report includes material from Bloomberg News.