Ms. Chao, reading from prepared answers at times, defended the F.A.A.’s decision not to follow the lead of 40 other countries by immediately grounding the fleet, despite the possibility that the Ethiopia crash might have resulted from a flaw in a plane whose basic design dates to the 1960s.

[Read more about how two high profile crashes led to a corporate crisis at Boeing.]

The F.A.A., which Ms. Chao oversees, waited three days to ground the fleet and was one of the last major regulators worldwide to do so. She flew back to Washington from a festival in Texas on March 12 on a 737 Max, in what seemed to be an endorsement of the plane’s safety. Less than a day later the F.A.A. uncovered evidence that suggested the planes were not fit to fly.

“The F.A.A. saw no basis upon which to ground these planes,” she said. “It is a very technical organization. It is very data-driven. They saw no data until the morning of Wednesday the 13th.”

It was at that time, she added, that F.A.A. investigators discovered “new information on the first three minutes of the Ethiopian” flight that revealed “parallel conditions” involving the two accidents.

But in an interview after the hearing, Ms. Chao made it clear that the decision to ground the jets was the F.A.A.’s — not her own — and said that she had no legal say, then or now, in deciding when the planes fly.