NAIROBI, Kenya — A senior engineer with Boeing says the company may never know what caused battery problems that have grounded the company’s 787 Dreamliners.

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

Horigan also said that all potential root causes of the battery fire have been eliminated with the new redesigned battery system.

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.

Boeing assembles the 787 at factories in NOrth Charleston and Everett, Wash.