I've read them with growing apprehension. Given the evidence uncovered, the use of these planes is definetly in play.
And, as Larousse2 notes, "Don't rule out those innocent looking remote control planes."
Hell, if the bastards want to go really low-tech, a big box-kite in a field somewhere could release a sizable dose of the death-stuff into a strong, prevailing wind.
These suicidal maniacs might be willingly infected with a horrible germ or virus: smallpox, ebola etc, then let loose in crowded airports etc to infect as many as possible.
Some of these, such as anthrax, have a fairly long infectious stage before symptoms set in, long enough to set off a chain reaction from airport to airport to airport.