Not at all hard to make a fairly lethal bomb the size of a toner cartridge, or even an ink cartridge. Depends on what the 'load' is, you know? Bio, nuclear, chemical...they don't need a whole lot to be fairly destructive, at least in a terrorist sense.
I mean, if a small bio weapon went off, and killed 200 people, that's not a huge death count, but imagine how scared anyone would be to open packages, or - shoot, anything?
Al Quaida (radical muslims - there, call me a racist if you will...) in Yemen, and AQ in general, are definitely active. They have, in the past, timed their attacks to coincide with specific dates and/or events, like elections and holidays. This timing is rather expected nowadays...
And if they target something that has an economic impact right now, it will be devastating in many ways. Imagine if the entire holiday shipping schedule was interrupted, either by additional security measures or by bombs exploding. How would that affect the holidays, especially when we're shipping things all over the world, importing tons of stuff, and so on? If we can't ship, shopping will drastically decline. If stores can't ship into their markets, shopping will drastically decline as well.
And since we are in an economic earthquake right now, the ripple effect on Wall Street, Corporate America, and the Mom and Pop stores who rely on holiday shopping to make ends meet will be in dire straits. Then add the trickle down theory, and we indeed have the makings of a serious attack, using only cargo planes and small bombs, with a low body count.
There have been many terrorist plots discovered and dismantled. As Fran said upthread, AQ seems to have been testing the waters for something bigger. AQ does, of course, want bigger, but even smaller and successful would work. I have no doubt that today's events are part of a testing process, and AQ has learned a whole lot today - making them even more deadly than they have been. There have been dry-runs (as Fran mentioned) with bomb making parts; watches, cellphones, and bits of relatively innocuous things which, together, are the pieces of the bomb puzzle. And I have no doubt that AQ wants to nail us again. The only questions I have are "when" and "how."
Seems to me, the "how" is being answered now.
The only question left, then, is "when."
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Herding Cats