Is al-Qaeda capable of bombing the United States?

Do they have connections to WMD or not? I see it going back and forth, people say they don't have the connections, money, etc, and some say that they are capable of doing it. They said to expect something huge, much greater than 9/11 and to me that says nuclear warfare. Also, does the US have ways of detecting this, preventing it, stopping it? Where would they most likely drop it if they did it? And how much damage (radius) would it cover/do?