On Friday, Boeing Co. gave U.S. aviation regulators its plan to fix the volatile battery aboard its new 787 Dreamliner, even though investigators have not yet determined what caused the batteries to overheat on two planes last month.

Boeing did not propose abandoning the lithium-ion batteries and is not working on a backup or longer-term fix for the problem that has grounded its entire fleet of 50 Dreamliners for nearly five weeks, three sources familiar with the plan said.

The company and the U.S. Federal Aviation Administration said no firm result emerged from the meeting between Deputy Transportation Secretary John Porcari, FAA Administrator Michael Huerta and other FAA officials and Boeing Commercial Airplanes CEO Ray Conner and other senior Boeing executives in Washington.

With Boeing’s costs mounting by millions of dollars a day while the planes are on the ground, the FAA said it is “reviewing a Boeing proposal and will analyze it closely. The safety of the flying public is our top priority and we won’t allow the 787 to return to commercial service until we’re confident that any proposed solution has addressed the battery failure risks.”

Boeing declined to comment on the details of its proposal, but said the meeting with the FAA was productive.

The proposal to the FAA includes measures to address a range of possible causes of short-circuits in the batteries, the sources said.

Five weeks ago, U.S. authorities grounded the worldwide fleet of 787s. U.S., Japanese and French investigators are still not certain what caused the battery fire aboard an All Nippon Airways 787 in Boston and an overheated, smoking battery on a Japan Airlines 787 in Japan.

The proposed fix includes adding ceramic insulation between the cells of the battery to help keep cells cool and prevent a “thermal runaway” in which one cell overheats and triggers overheating in adjacent cells. It also includes building a stronger, larger stainless steel box with a venting tube to contain a fire and expel fumes outside the aircraft should a battery catch fire again, the sources said. In addition, the plan proposed wiring changes, self-torquing screws that will not come loose and battery alterations to prevent moisture and vibration problems, one of the sources said.

But there was also a plan to use a different battery type or some other longer-term fix, the sources said.

“I have talked to a number of people who are working directly on these batteries. No one is on the Plan-B team,” said a person familiar with Boeing’s efforts who was not authorized to speak publicly about them.

A second source, who also was not authorized to speak publicly, said Boeing does not view its proposal as a temporary “band-aid” that would be supplanted by another solution later.

Boeing spokesperson Marc Birtel said in a statement: “We are encouraged by the progress being made toward resolving the issue and returning the 787 to flight for our customers and their passengers around the world.”

The U.S. National Transportation Safety Board is still investigating the Boston fire and the Japan Transportation Safety Board is investigating the battery failure in Japan. Neither has found a root cause for the problems.

The sources said the NTSB might never find the cause because the battery in Boston was severely damaged by the fire.

Given the financial cost of the grounding for Boeing and the airlines that own the jets, estimated at $200 million a month, Boeing decided to address all possible causes with the measures, rather than wait for the NTSB to identify one specific cause, the sources said.

Boeing engineers have been working with outside experts and U.S. government officials to address possible cause of the battery issues. The team includes experts from the U.S. Navy and the National Aeronautics and Space Administration, which uses a lithium-ion battery on board the International Space Station.

Boeing engineers went through a “fault tree” and “came up with a list of half a dozen things that could have led to problems,” said a congressional source who had been briefed on the matter, but was not authorized to speak publicly.

“They have a list of things that it could be, and the fixes are designed to address that list of problems,” the source said.

If the NTSB’s investigation turns up additional possible causes, those would be added to the mix, another of the sources said.

Asked why the company’s extensive testing of the batteries had not revealed problems with the batteries and the electrical systems used to operate them, one of the sources said test environments had limitations and the real test of an aircraft always came when it was actually operating.

If the Boeing plan is approved by FAA Administrator Huerta and Transportation Secretary LaHood, company officials expect the 787 fleet to return to service within eight weeks, one source said.