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The passenger gave the weapon to a member of the flight crew, who returned it to the air marshal. But the marshal, who is based in the New York region, failed to report the incident to her superiors, as required by agency policy, until several days later. The incident happened on April 6, aboard a Delta flight from Manchester to Kennedy International Airport.

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The air marshal who left the weapon unattended, according to people with knowledge of the situation, is a new hire. The T.S.A. declined to provide any additional information.

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Current and former employees said the episode raised questions about how discipline is handled at the agency. The employees said some air marshals had been forced to resign or had been fired for minor transgressions. “It's a toxic culture and a lack of accountability,” Mr. MacLean said.

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