An American air marshal has shot and killed a passenger at Miami airport in Florida.
The extact number of US air marshals is classified
The incident marks the first time a passenger has been killed by marshals since the 11 September 2001 attacks on the US.
BBC News summarises their role and the issues surrounding their deployment.
What are air marshals?
An air marshal is an undercover armed guard on board a commercial aircraft, to counter hijackings and other hostile acts.
US marshal training includes marksmanship, intimidation tactics and emergency procedures, according to the department for homeland security.
What is the US policy on air marshals?
There were 33 federal air marshals at the time of the 2001 attacks. The Bush administration hired thousands more afterwards, but the exact number is classified information.
The programme began in 1968 and was expanded in 1985. The service is now part of the department of homeland security.
In late 2003 the US said international flights to or over the country would be required to carry armed guards in certain cases. Washington said flights could be banned if airlines refused to comply.
The demand prompted concerns over safety and cost from some carriers.
Some were worried that a firearm carried by a sky marshal could be used against the aircraft or its crew.
How many other nations use air marshals - and what is their record?
Israel's El Al has had armed marshals operating on its flights for more than 30 years.
A minority of other countries have also introduced armed guards on board, including Germany, Canada and Australia.
In the 1970s a team managed to overpower hijackers on an El Al flight from Amsterdam to New York by warning the pilot, who put the plane into a steep dive, throwing the attackers off their feet.
In 2002 El Al guards arrested a man with a knife who appeared to be storming a cockpit.
Also in 2002, two hijackers on an Ethiopian airline flight were shot.
Are any guns safe on aircraft?
There will always be the risk of injury, but the level of risk is thought to be manageable.
There is the potential for gunshots to penetrate the cabin walls, but this is unlikely to lead to serious cabin decompression.
A missing window can be compensated for by the plane's air supplies, and pilots are trained to descend should cabin pressure be affected.
Aircraft must also carry enough fuel to divert to a nearby airport.
There is the risk that gunfire will hit critical systems, but air marshals are trained to watch the background behind their target.
They also use what is called "pre-fragmented ammunition", which is designed to break apart on impact, not pass through the body.
Nevertheless, a gunfight on a passenger airliner is to be avoided at all cost.
What is the consensus among security experts on air marshals?
Experts in general say there is nothing wrong with them in principle, but that other security measures are more important.
Security is a chain stretching from the point where passengers arrive at the airport, to the point where they leave at their destination.
Stopping a potential attacker before they board the plane is the priority, though many experts believe air marshals are an important last line of defence.