Hmm... nothing happened... strange.

Theres 3 reasons I can think of, off the top of my head -

1. Lulling us into a false sense of security. People expected something to happen in Chicago, so the BA doesnt do anything.
2. The BA isnt behind the attacks, it is, in fact, terrorists that are in no way linked or affiliated with the American government.
3. It was a front for something else. As this was going on, the BA/terrorists planted bombs, nuclear weapons, dirty bombs etc to use in future attacks.