WASHINGTON – Firefighters and mechanics tried repeatedly to put out a battery fire aboard a Boeing 787 Dreamliner through smoke so thick 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 International Airport is under investigation by the National Transportation Safety board, which released laboratory analyses, interviews and other data it has gathered so far. It still hasn’t been able to pinpoint the cause.

Federal Aviation Administration officials are expected to make a decision in the next few days on whether to approve a plan by Boeing to revamp the 787’s lithium ion batteries to prevent or contain future fires. Once the plan is approved, Boeing hopes to swiftly test the reconfigured batteries and get the planes back in the air.