U.S. air travel returned mostly to normal Thursday, a day after a computer system that sends safety information to pilots broke down and grounded traffic from coast to coast.

By midafternoon on the East Coast, about 150 flights had been canceled and more than 3,700 delayed — much lower figures than on Wednesday, when more than 1,300 flights were scrubbed and 11,000 delayed.

Attention turned to the federal agency where the technology failure apparently started hours before it inconvenienced more than 1 million travelers.

The Federal Aviation Administration said a damaged database file appeared to have caused the outage in the safety-alert system. Transportation Secretary Pete Buttigieg promised a thorough examination to avoid another major failure.

Buttigieg said there was no indication that the outage was caused by a cyberattack but that officials would not rule that out until they know more.

The FAA said late Thursday that a preliminary analysis showed the breakdown came after “a data file was damaged by personnel who failed to follow procedures.”

The massive disruption was the latest black eye for the agency, which has traded blame with airlines over who has inconvenienced passengers more. Critics, including airline and tourism leaders, say agency technology is underfunded.