[Mr. Jeeves]

Most analysts think that while Al Qaeda may have enough material from old weapons to create a dirty bomb, they don't actually have a working nuclear device.

Of course, most analysts thought 9/11 was impossible, too...

[griswold3]

Read this report:
http://www.nti.org/e_research/e5_publications_Nuclear%20Terrorism.html
Aum Shinrikyo, Al Qaeda, and the Kinshasa Reactor: Implications of Three Case Studies for Combating Nuclear Terrorism
Sara A. Daly, John V. Parachini and William Rosenau, RAND Corporation, April 2005
View report

Revelations about A.Q. Khan’s global nuclear marketing efforts and Osama bin Laden’s contact with Pakistani nuclear scientists have raised concerns about terrorist acquisition of a nuclear or radiological weapons capability. Such a capability would pose a grave danger to U.S. national security and to the security of the international system of nation-states. This study suggests that strict controls on nuclear weapons, materials, and expertise will reduce opportunities for terrorists to acquire these resources.

[The_Reader_David]

No, analysts didn't think, 9/11 was impossible: analysts didn't have the vision to think one way or another about the feasibility of such a plot.