You'll note that the majority of the leading economic indicators presently show a growth economy. The major indicator that is contrary is the stock market.
9/11 obviously did have an effect, but an over priced stock market, (P/E ratios at near all time highs), poor power market deregulation laws in Calif, (they deregulated wholesale, but not retail and wrote loopholes to be exploited), the realization of the "tulip ecconomy" of the internet sector, (read your dutch history), these all happened at near the same time, and exerted a great driving force to the downturn.
Of course if you were heavily invested in UAL, 9/11 was a biggy.
The stock dump would have happened regardless of Osama.