WASHINGTON (MCT) — The top two federal regulators who grounded Boeing's 787s over battery problems vigorously defended their decision Wednesday amid continuing uncertainty about when the jetliners will be cleared to fly again.

Facing a scrum of reporters for the first time since issuing a rare order Jan. 16 to pull the Dreamliner out of service, Secretary of Transportation Ray LaHood and Michael Huerta, the administrator of the Federal Aviation Administration, said the 787 won't return to the air until investigators find the root causes of the malfunction.

"We must be confident that the problems are corrected before we can move forward," LaHood said in Washington.

LaHood and Huerta issued the emergency directive following separate incidents earlier this month in which one high-energy lithium ion battery caught fire in a parked plane and another sprayed overheated electrolytes, forcing an emergency landing. After the second incident, Japan Airlines and All Nippon Airways, the two carriers involved, voluntarily grounded their new 787 fleets. The FAA followed suit hours later.

Sounding combative at times, LaHood, a former congressman, fended off questions about his and Huerta's technical expertise, criticisms that the FAA should have grounded the planes sooner after the first fire, and questions on whether doing so after the second incident was overkill.

LaHood said the grounding was necessary to ensure the plane was safe, and that a thorough review by technical investigators will determine the 787's airworthiness.

"We need to let them finish their work," LaHood said. "They'll get to the bottom of it."

Asked if there was pressure to lift the flight ban, LaHood said, "Absolutely not. Boeing is cooperating 100 percent with this review."

Huerta added, "We don't know what caused these incidents yet." He said any corrective steps — and the lithium ion battery's fate — will be dictated by evidence.