WASHINGTON — A federal air marshal on a flight earlier this month from England to New York left her loaded service weapon in the aircraft’s bathroom where a passenger found it, four marshals familiar with the incident said.
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.
Despite the security lapse, the marshal was assigned to a flight a few days later, people familiar with the case said.
The Transportation Security Administration, the parent agency of the air marshal service, said that it was aware of the episode but that it would not comment publicly on internal matters, adding that it was “reviewing the circumstances of this incident.”
Current and former air marshals said that leaving a loaded weapon unattended constituted a significant security breach that should have resulted in discipline and an investigation.
“You can’t have inept people leaving weapons in a lavatory,” said Craig Sawyer, a former air marshal. “If someone with ill intent gets hold of that weapon on an aircraft, they are now armed.”
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.
The disclosure comes after officials received intelligence showing that Islamic State militants are actively trying to target aircraft, including by hiding bombs in electronic devices. That revelation led officials in the United States and the United Kingdom to bar passengers from airports in 10 Muslim-majority countries from carrying laptop computers, iPads and other devices larger than a cellphone aboard direct inbound flights.
The episode is the latest in a string of embarrassments that have plagued the air marshal service in recent years, including allegations of sexism and racism and of employees arranging their schedules to meet up for sexual trysts.
In one of the most high-profile cases, the agency fired Robert MacLean, a federal air marshal, for disclosing to a reporter the agency’s plans to reduce the number of air marshals on flights. Mr. MacLean sued the agency, and the whistle-blowing case went all the way to the United States Supreme Court, which decided in his favor in 2015.
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.
Several thousand air marshals in plainclothes sit anonymously among passengers and provide the first line of defense against Sept. 11-style terrorist attacks or other threats against jetliners.
The service was started by President John F. Kennedy in 1961 to protect against hijackings.
This month’s incident could provide ammunition to congressional critics of the air marshal program, who say that it is wasteful and unnecessary. The program accounts for about 10 percent of the T.S.A.’s budget, costing nearly $1 billion per year. Critics say it is unclear that the program has actually deterred a terrorist attack.