Airline sources say that's common practice -- and it does put them close to the cockpit door. But it's more complicated than that:

With cockpit doors fortified and a history of attackers choosing coach seats, some airline executives and security experts question whether the first-class practice is really necessaryor even a good idea. It could weaken security by isolating marshals or making them easier for terrorists to identify, airline executives say.

With more threats in the coach cabin now, first-class clustering may not make as much security sense. Security experts say bombers are a bigger threat today than knife-wielding attackers trying to get through secure cockpit doors, and Transportation Security Administration checkpoints are heavily focused on explosives, whether hidden in shoes, liquids or under clothes. Some believe bombers try to target areas over the winga structurally critical location and also the site of fuel storageto cause the most damage to the aircraft.

There's also this:

By law, airlines must provide seats to marshals at no cost in any cabin requested. With first-class and business-class seats in particular, the revenue loss to airlines can be substantial because they can't sell last-minute tickets or upgrades, and travelers sometimes get bumped to the back or lose out on upgrade opportunities. When travelers do get bumped, airlines are barred from divulging why the first-class seat was unexpectedly taken away, to keep the presence of a marshal a secret.

So airlines have a financial incentive to pressure air marshals out of first class, and the marshals themselves may be influenced by the temptation of comfy seats. It's hard to know who to trust on this one.

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