U.S. aviation regulators have no timeline for returning Boeing Co.’s grounded 737 Max to service and won’t act until they are sure it is safe, the nation’s top transportation official said Thursday.

The Federal Aviation Administration has to be assured that a fix being developed by Boeing in the wake of two fatal crashes will prevent any future accidents, Transportation Secretary Elaine Chao said in a speech in Washington.

“The FAA will lift the aircraft’s prohibition order when it is deemed safe to do so,” Chao said. “That is the bottom line: There is no timeline.”

Chao was speaking before the Air Line Pilots Association’s Air Safety Forum. ALPA is the largest pilot’s union in North America.

Boeing is altering software on the plane that had malfunctioned in both accidents, pushing each plane’s nose down without pilot input. The crews weren’t able to counter the plane and they lost control.

FAA is also developing new training requirements for pilots on the 737 Max, Chao said. A technical advisory board including experts from NASA and the Air Force is also weighing in on the decision, Chao said.

Her comments mirror what FAA acting Administrator Daniel Elwell has been saying in recent months.

The attempt to adapt the software on the Maneuvering Characteristics Augmentation System, which has been identified as a factor in crashes in Indonesia and Ethiopia, has been slower than was initially predicted.

In recent weeks, the FAA identified a new potential flaw linked to how the plane responds during a failure of a flight computer on the plane. Boeing has said that it can address the problem with a separate software patch.

The planemaker has been telling customers and other stakeholders that it will complete its package of fixes and flight tests and submit it to the FAA by September.

Airlines have been canceling flights. Southwest Airlines Co., a major Max customer, announced this week that it is removing the Max from schedules through Nov. 2, about a month later than previous date of Oct. 1.