A passenger found the gun in the bathroom and turned it over to crew members, who returned it to the marshal. The April 6 Delta flight was from England to New York City.
The Transportation Security Administration said it was aware of the situation and that it was “reviewing the circumstances of this incident.”
The marshal failed to report the incident until days later, according to The New York Times, but she was assigned to another flight a few days after the incident.
The situation should have resulted in discipline and an investigation, current and former air marshals said.
“You can’t have inept people leaving weapons in a lavatory,” Craig Sawyer, a former air marshal, told The New York Times. “If someone with ill intent gets hold of that weapon on an aircraft, they are now armed.”