Post 911 studies of al Qaeda and the way they operate indicate that luck (among the terrorists) and mistakes (by anti-terrorism organizations) were mainly responsible for the success of the September 11, 2001 attacks, and earlier ones as well. Al Qaeda operatives, although they have some training in secret operations and sabotage, were basically amateurs, and made many mistakes. Too many of these errors were not detected by police and counter-terrorism experts. Since 911, al Qaeda has lost many of its most experienced people to death or capture. Most al Qaeda training camps were shut down, choking off the supply of new operatives. However, killing three thousand Americans in 2001 inspired thousands of young Moslems to try and do the same and, well, it's unclear exactly how all this arbitrary murder will improve anything. That, however, is a positive thing for counter-terrorism forces. As long as the terrorists are ignorant and obsessed, they have to depend more on chance than deliberate action, to carry out successful attacks. 

In the last three years, counter-terrorism forces have put more agents into al Qaeda organizations, and tapped into the Internet communications network that keeps al Qaeda in touch with itself. This has not indicated that much of an al Qaeda "central headquarters" remains in operation.  Still a very decentralized organization, al Qaeda keeps itself moving with lots of mutual pep talks and encouraging rumors. It's still unclear if any of this "chatter" can indicate a coming attack. But to be on the safe side, targets getting the most "chatter" are sent warnings. Thus the recent warnings for Americans to get out of Bahrain, the island country off the east coast of Saudi Arabia that allows alcohol and other activities condemned by Islamic radicals.