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

Transportation Secretary Ray LaHood stood by his Jan. 11 assertion that the 787, Boeing’s newest and 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 ― including Chile, Poland, Ethiopia, Qatar and India ― swiftly followed suit. Two Japanese airlines voluntarily grounded their planes before FAA’s order.

Overall, 50 Dreamliners have been grounded worldwide. FAA’s order applies only to United’s the six 787s.

“On the day we announced the planes were safe they were,” LaHood told reporters 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 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 already 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 is also 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 so far said the battery problem would endanger the safety of the plane in flight nor recommended that the planes be grounded.

The board’s technical experts are in possession of the battery that caught fire and are effectively performing an autopsy on its charred insides in a search for clues to what caused the conflagration. It took firefighters about 40 minutes to put out the fire.

The NTSB is the nation’s independent accident investigation board, while FAA regulates aviation safety.

FAA is working as quickly as possible to find the cause of the problems, assembling a team of technical experts that includes experts from industry as well as the agency’s staff, Huerta said. The review includes not just the 787’s ground-breaking lithium-ion battery system, but how that system works with the aircraft’s electronic systems, their certification, manufacture and assembly, he said.

Huerta declined to say when FAA might lift the grounding order.

“We don’t know yet what caused these incidents yet. When we know the cause we will take appropriate action,” he said.

The officials emphasized that the investigation would be completely transparent so that the public will have confidence in the outcome.

LaHood denied that Boeing had asked the government to lift the grounding order.

“Absolutely not,” he said. “Boeing is cooperating 100 percent with the review.”

The groundings have been a nightmare for Boeing, which competes with Airbus for the position as the world’s largest commercial aircraft maker. At the time of the groundings, Boeing had orders for more than 800 of the planes from airlines around the world attracted by the 787’s increased fuel efficiency. The aircraft maker has said it has stopped delivering new planes to customers, although it is continuing to manufacture them.

The 787 is the world’s first airliner whose structure is made mostly from lightweight composite materials. It also uses electronic systems for most of its functions instead of hydraulic or mechanical systems. And it is the first airliner to make extensive use of lithium ion batteries, which are lighter, can hold more energy and are easier to fit into odd-sized spaces in airplanes than other types of batteries.

The FAA certified the 787 battery system even though lithium ion batteries are more susceptible to catching fire when they overheat or short-circuit than other types of batteries. Boeing built several safeguards into the design of the battery system.