If one takes a look at what these links turned up, then you will have a better idea of how dangerous the threat is.
The information is out there, a lot will be in our and other governments papers.
Interesting information in these searches:
http://www.google.com/search?client=googlet&q=sleeper%20cells%20in%20U.S.
http://www.google.com/search?client=googlet&q=why%20hasn%27t%20al%20Qaeda%20used%20WMD%20on%20the%20United%20States%3F
http://www.google.com/search?q=will+al+Qaeda+attack+the+United+States+with+WMD%3F&btnG=Search&hl=en&lr=&ie=ISO-8859-1&client=googlet
http://www.google.com/search?q=will+al+Qaeda+use+WMD%3F&btnG=Search&hl=en&lr=&ie=ISO-8859-1&client=googlet
http://www.google.com/search?q=did+al+Qaeda+hire+WMD+scientists%3F&btnG=Search&hl=en&lr=&ie=ISO-8859-1&client=googlet
http://www.google.com/search?q=will+al+Qaeda+use+WMD%3F&btnG=Search&hl=en&lr=&ie=ISO-8859-1&client=googlet
http://www.google.com/search?q=who+sold+al+Qaeda+WMD%3F&btnG=Search&hl=en&lr=&ie=ISO-8859-1&client=googlet
http://www.google.com/search?client=googlet&q=who%20sold%20al%20Qaeda%20nuclear%20weapons%3F
http://www.google.com/search?q=does+al+Qaeda+have+nuclear+weapons%3F&btnG=Search&hl=en&lr=&ie=ISO-8859-1&client=googlet
http://www.google.com/search?client=googlet&q=do%20terrorists%20have%20nuclear%20weapons%3F
http://www.google.com/search?q=where+to+buy+nuclear+weapons+today&btnG=Search&hl=en&lr=&ie=ISO-8859-1&client=googlet
http://www.google.com/search?client=googlet&q=where%20to%20buy%20nuclear%20weapons%20
http://www.google.com/search?client=googlet&q=nuclear%20weapons%20black%20market
http://www.google.com/search?client=googlet&q=missing%20nuclear%20%20weapons%20United%20States
Saving for in-depth reading... Thanks Ruth.