Al Qaeda proper is focused, rightly from a strategic perspective, on hollowing out Pakistan. So if there is a terrorist attack on US soil, from which hub of the open source insurgency will it come from? The strategist Tom Barnett makes the call:
The group involved, al Shabab, is described as a post-9/11 version of al Qaeda: loosely coupled cells and very decentralized. The ties between this group and al Qaeda go way back, and "many in the current leadership cadre are graduates of al Qaeda training camps." AQ vets make up a significant portion of al Shabab's top talent.
Then we hear about the wormholes that link Somalia to Minneapolis, Stockholm, Cardiff and Dubai... When the attack finally comes, the evidence trail leading up to it will be vast.
Following global guerrilla warfare theory, here's some quick analysis. There's been a long enough pause since 9/11 to generate a great deal of psychological shock from a significant new attack.
NOTE: since the shock value from strategic "blood and guts" terrorism suffers from diminishing returns -- new attacks must either be a) bigger than the first attack or b) delayed until the initial shock wears off -- in order to generate a similar or better shock than the earlier attack. If a group doesn't follow these rules, they appear diminished and lose brand value.
The attack, if it does come, will likely be suicide bombers (inexpensive wetware that is employed as terminal guidance systems for explosives) that detonate their packages in crowded commercial areas or Mumbai/Kabul style rampages/hostage take overs. If there is an attack on commercial aviation, it will likely move up the network to the terminal hub area rather than a low yield focus on aircraft. Breadth and simultaneity will likely be substituted for depth (meaning more places rather than bigger booms), to achieve maximal shock effect.
Unfortunately, the US is suffering from a new window of vulnerability to this type of attack. Unlike a year ago, any attack on US commercial areas (retail, transportation, etc.) will have outsized network effects. Here's why. Due to a global economic collapse and excessive indebtedness, Americans have cut back on purchases to repair household balance sheets (this is a long running secular shift). This has put most retail facing firms on the edge of bankruptcy. Any attack on commercial crowds over a large geographic area would radically reduce already depressed revenues at these firms (and drive costs for security through the roof), as people stay away from crowds until they feel safe again. So, in an unusual turn of circumstance, blood and guts terrorism is now closely aligned with the multiplicative effect of economic systems disruption. As a result, the economic damage to the US from an attack of this type as it struggles to ward off economic depression, would be vast. Firms would wink out of existence at an alarming rate. Returns on investment (ROIs) from any attack of this type could potentially reach $100 million in economic loses for every $1 in attack costs.