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A federal air marshal on a transatlantic flight left her loaded service weapon in the airplane’s bathroom, where it was discovered by a passenger who gave it to a crew member before it was returned to the federal agent, CNN has learned.

The incident happened aboard Delta flight 221, which was traveling from Manchester, United Kingdom, to New York’s John F. Kennedy International Airport on April 6, and was reported to the air marshal’s management days later.

In a statement to CNN the TSA would confirm only that it is aware of an incident involving a federal air marshal on that date, and it is reviewing the circumstances.

A Delta spokesperson said the airline is also aware of the incident but declined to provide further comment.

Sources within the Federal Air Marshal Service said the agent in question is a relatively new hire who should have been placed on leave for leaving her gun. Instead, the agent remains on active flight duty, CNN has been told.

A former federal air marshal familiar with the situation told CNN that newly hired air marshals do not currently receive on-the-job training and said this incident might have been avoided if the individual had been paired with a senior air marshal.

“She made a mistake because she wasn’t given the appropriate tools to succeed,” the former air marshal said.

John Casaretti, president of the Air Marshal Association and a former air marshal, told CNN the TSA should implement additional training for newly hired air marshals.

“These rare incidents must be thoroughly investigated and local managers should take appropriate corrective action,” Casaretti said. “A field training officer program and thorough mentorship of new officers can reduce similar performance issues.”

“Air marshals work in punishing conditions, labor under poor leadership and have seen their law enforcement functions curtailed by an administration that lacks vision. The problem is not the air marshals, it’s the TSA,” he added.

The Federal Air Marshal Service has long been criticized for poor management and lack of proof that what they do — fly undercover to monitor for and respond to threats — significantly improves security. CNN has been unable to find a single incident in which a federal air marshal deterred or intervened in a terrorist plot since the organization was ramped up after the September 11, 2001, attacks.

Although it has an $835 million budget, agents cover less than 1% of US domestic and international flights, sources familiar with the agency’s operations said.

A CNN report in 2015 exposed the long hours, chaotic schedules and use of drugs and alcohol among federal air marshals, which has led to suicides and suicide attempts by the agents.

In that report, CNN obtained a now-classified study commissioned by the TSA that revealed 75% of air marshals flying domestic missions were sleep-deficient. The percentage was even higher on air marshals assigned to international assignments, where 84% of agents were identified as sleep-deficient.

According to the 2012 study conducted by Harvard Medical School’s division of sleep medicine, “the acute and chronic lack of sleep substantially degrades a federal air marshal’s ability to react and think quickly.”

TSA officials insisted at the time that air marshals’ schedules ensure appropriate rest periods and that agents have access to a “robust health, fitness and wellness program.”