A Boeing engineer said Thursday that the cause of the battery problems that have grounded the company's 787 Dreamliners may never be known.

The engineering leader for the 787, Richard J. Horigan, said the root cause of smoldering batteries experienced by the two different 787s may never be known because the evidence was destroyed by heat.

“When we say a root-cause we want to know exactly what happened, what size particles caused the cell to vent ... and we may never get there because the evidence is destroyed. When these cells vent there's a lot of heat damage on those cells,” Horigan said.

Horigan also said that all potential causes of the battery fire have been eliminated with the new redesigned battery system. He said it was not the first incident where a problem that caused a malfunction in an aircraft has not been identified definitively, but a solution covering the problem had been found.

Horigan cited the 1996 crash of TWA 800. The plane crashed after a fuel tank explosion, but the cause was never identified.

After TWA Flight 800 exploded off the coast of Long Island, New York, in 1996 killing all 230 people aboard, investigators ultimately pinned the cause on flammable vapors in one of the Boeing 747's fuel tanks. Investigators were never able to conclusively identify the source of the spark that ignited vapors

Horigan said in the case TWA 800 plane experts came up with a solution that “fully encapsulated and eliminated all possible causes of fuel tank fire.”

“So it's not uncommon where you have events like this where you don't get a definitive root-cause but you have sufficient confidence in your design solution to know that whatever the root cause is, you've identified it and addressed it,” he said.

Air safety authorities grounded Boeing 787s after incidents with smoldering batteries occurred aboard two different planes in January. The U.S. Federal Aviation Administration has approved Boeing's redesigned battery system, which the company says sharply reduces the risk of fire. 

Once the FAA approves the fix on individual planes, airlines can start flying them again. United Airlines, the only U.S. airline with the planes, moved one of its six 787s to a Boeing facility in San Antonio, Texas, on Tuesday so it can get the battery fixed. Neither of the battery incidents involved a United jet.

Boeing said Wednesday that deliveries of the 787 should resume in early May. Most of the 50 planes that have been delivered to airlines will be fixed by the middle of the month. (AP)