Washington (CNN) -- When firefighters responded to reports of smoke on a new Boeing 787 Dreamliner parked at Boston Logan International Airport on January 7, they found smoke billowing from the cabin floor.
A mechanic directed them to the plane's aft electronics bay.
There, one firefighter, using a thermal imager to see through thick smoke, saw a "white glow" the size of a softball on the front face of the battery box. Hot liquid oozed out of the rectangular box like "candle wax," he told investigators.
When firefighters knocked down the small fire, it would rekindle. One described the battery as sizzling.
Those descriptions, gathered soon after the incident, were released Thursday as the National Transportation Safety Board published an interim report into the fire aboard the Japan Airlines Dreamliner.
They were among the first indications that Boeing's new lithium ion battery had failed some of the nine special conditions set by the FAA -- conditions that, in short, require batteries be designed against failure, and containers designed to contain damage if they do.
Two months after the incident, and seven weeks since the Federal Aviation Administration grounded the Dreamliner, officials have yet to publicly explain the exact cause of the battery failure, or of a battery failure a week later on a Dreamliner flying over Japan.
The NTSB, the FAA and Boeing have all said they have made significant strides in investigating the incident. And Boeing said it has given the FAA its plans to test possible solutions.
The federal government has set a high bar in returning the plane to the sky. Transportation Secretary Ray LaHood has said he will approve flights only when he is "1,000%" confident in the aircrafts' safety.
The NTSB said it will hold two hearings in April -- one, a forum on the use of lithium ion batteries in transportation, and another investigative on the B-787 battery issue. Together, the hearings will help the entire transportation community "better understand the risks and benefits" of lithium batteries, NTSB Chairwoman Deborah Hersman said in a statement.
When Boeing unveiled its Dreamliner aircraft, press accounts focused on its cutting edge use of lightweight composite materials, its spacious cabin, large windows and fuel-saving engines. But the lithium batteries were also an innovative feature of the plane. They were lighter than the traditional nickel-cadmium and lead-acid rechargeable batteries used on previous jetliners, and they packed more power.
The batteries -- one in the front of the plane, one in the rear -- power the aircraft's main and auxiliary power units, flight control electronics and emergency lighting system.
But in general, the FAA says, lithium ion batteries are significantly more susceptible to internal failures that can result in thermal runaways, or self-sustaining increases in temperature and pressure. Both overcharging and overdischarging can cause problems, and lithium battery fires are more difficult to extinguish than other batteries, experts say.
The NTSB says that in seeking certification of its batteries, it estimated that a "smoke" event would occur "less than once in 10 million flight hours." But after 51,662 hours of commercial flight, two batteries failed, one culminating in a fire. In the Japan incident, damage was confined to the battery housing.
Further, Boeing's indications that heat damage in one battery cell would not harm adjacent cells proved false, Hersman said. "The assumptions used to certify the battery must be reconsidered," she said.
The plane in the Jan. 7 incident was new -- having only flown 22 commercial flights, the NTSB said. It had completed 6.5- or 7-hour flight from Japan, and had been emptied of passengers, when the first signs of trouble appeared. The planes' auxiliary power unit, powered by the aft battery, suddenly stopped working, killing electricity to the aircraft. Shortly thereafter, the cleaning crew and airline workers smelled smoke and called for help.
One firefighter received minor injuries. It took firefighters more than an hour to extinguish the fire and remove the battery. The NTSB said it is continuing its investigation.
Boeing has delivered 50 Dreamliners worldwide, including six to a U.S. airline, United.