Nerve Agent? Why bother boarding a flight? All they have to is expose it at the airport, look at the crowded terminals.
There was a 24 episode last season about terrorists using a nerve gas canister to kill a docked submarines crew and take over the submarine. They may have had masks or some other means to counter the nerve agent, like for example injecting the antidote before boarding the plane or while in the restroom. Just guessing.