Operations had resumed by Monday evening, with delays. The incident revived concerns about the Federal Aviation Administration’s aging air traffic control infrastructure.

© Dakota Santiago for The New York Times

Scenes from extra long lines at two airports, and a surprising discovery at a third.

© Alyssa Pointer/Reuters

The agents were sent on Monday to help understaffed T.S.A. teams manage long security wait times. But at many airports, their duties seemed limited.

© Vincent Alban/The New York Times

The National Transportation Safety Board said that it was investigating a “close call” that happened as the planes were landing on Tuesday evening.

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