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...