WASHINGTON —Obama administration officials struggled Wednesday to defend their initial statements that the Boeing 787 Dreamliner is safe while promising a probe of mishaps involving the aircraft’s batteries.

Transportation Secretary Ray LaHood stood by his Jan. 11 assertion that the 787, Boeing’s most technologically advanced airliner, was safe. At that time, LaHood and the head of the Federal Aviation Administration, Michael Huerta, declared the plane fit to fly despite a battery fire in one plane.

Five days later, following another battery mishap that led to an emergency landing of a 787 in Japan, LaHood and Huerta ordered United, the lone U.S. carrier with 787s, to ground the planes. Authorities in Europe and elsewhere swiftly followed suit. Two Japanese airlines voluntarily grounded their planes before the FAA’s order.

Overall, 50 Dreamliners made at Boeing plants in Everett, Wash., and North Charleston have been grounded worldwide. The FAA’s order applies only to United’s six 787s.

“On the day we announced the planes were safe, they were,” LaHood said at an aviation industry luncheon. He became testy when a reporter pressed him on whether his initial pronouncements had been too hasty.

“I’m not doing these hypothetical look-backs,” he said. “We did what we did.”

What changed between Jan. 11 and the FAA’s issuance of a grounding order on Jan. 16 was that a second battery failure occurred on an All Nippon Airways 787 while the airliner was in flight, said Huerta, who joined LaHood at the luncheon.

In the first incident, the battery fire occurred in a Japan Airlines 787 that had landed at Boston’s Logan International Airport and was empty of passengers. “We took the action we took (to ground the planes) because we saw a hazard,” Huerta said.

The National Transportation Safety Board also is investigating the battery fire in Boston and has sent a representative to Japan to assist authorities there with their investigation of the second.

The board has not said so far that the battery problem would endanger the safety of the plane in flight, nor recommended that the planes be grounded.

The board’s technical experts have the battery that caught fire and are effectively performing an autopsy on its charred insides in a search for clues to what caused the fire.