(Reuters) — A U.S. safety regulator on Thursday released hundreds of pages of details from its probe of a Boeing 787 battery fire, but the information did not reveal what caused the January blaze nor advance the Boeing Co.'s goal of getting the plane back in the air.

The U.S. National Transportation Safety Board said it would hold a public hearing in April on the design and certification of Boeing's 787 battery system and a forum on lithium-ion battery technology in general the same month.

"The information developed through the upcoming forum and the hearing will help the NTSB and the entire transportation community better understand the risks and benefits associated with lithium batteries, and illuminate how manufacturers and regulators evaluate the safety of new technology," NTSB Chairman Deborah Hersman said in a statement.

The NTSB is conducting an extensive review of the 787 battery failure, examining not just the burned battery carcass but the certification and testing of the system.

The battery caught fire aboard a parked jet shortly after it landed at Boston's Logan International Airport in January.

Japanese regulators are investigating a second incident that occurred during a flight in Japan nine days later. Regulators worldwide quickly banned the 787 from commercial flight after the second incident, a restriction costing Boeing and airlines millions of dollars.

The NTSB's 39-page "interim factual report," part of 499 pages of studies released on Thursday, provides extensive detail on the testing performed on the battery. But it also makes clear that investigators remain a long way from understanding why the battery caught fire in the first place.

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Among the report's findings: a system designed to vent smoke outside the plane during a battery fire failed to function because it lacked power after the battery caught fire. The system's auxiliary power unit (APU), a gas-driven engine in the tail of the plane, also was shut off at the time, and the battery is used to start that system.

"As a result, smoke generated by the APU battery could not be effectively redirected outside the cabin and aft (electrical equipment) bay," located in the fuselage behind the right wing of the 787.

Boeing had said that the venting system failed because the plane was on the ground and lacked cabin pressure to use in expelling fumes from the cabin.

The NTSB report, while marking a milestone in the probe of the Boston fire, also signaled that the NTSB has significant additional work to do in its investigation.

The NTSB said a group focused on system safety and certification was going through records of testing and analysis performed by Boeing and its suppliers, system maker Thales S.A. of France, and battery maker GS Yuasa Corp. of Japan.

The NTSB report also revealed abnormal function of the battery shortly before the fire was reported. It said a flight data recorder showed that the voltage and current in the battery fluctuated sharply, though never exceeded 32 volts, its rated level. Overcharging has been associated with fire.

In one instance, about 21 minutes after the plane landed, the battery voltage dropped to zero and rebounded to 28 volts three times in three seconds.

About three minutes later, a ground crew member "re-entered the cockpit and reported smoke in the cabin," the report said.