U.S. Transportation Secretary Ray LaHood said Friday that his department has no estimate as to when the 787 Dreamliner, manufactured by the Boeing Company (NYSE:BA), will be back in service.

The Federal Aviation Administration is investigating the cause of two incidents involving the jet’s lithium-ion battery unit. On Wednesday, an All Nippon Airways Co. Ltd. (TYO:9202) 787 had to make an emergency landing at Takamatsu airport after pilots noticed a burning smell.

The incident came only nine days after a fire broke out on a Japan Airlines Co. Ltd. (TYO:9201) 787 parked on the tarmac of Boston’s Logan International Airport. There were no passengers on the plane at the time, and no one was injured.

The FAA is currently working with Boeing engineers to identify the problem. Lithium-ion batteries can heat up if they are overcharged, posing a fire risk; however, the cause of the fires has not yet been determined.