In 1998, al Qaeda leader Osama bin Laden declared that acquiring and using weapons of mass destruction (WMD) was his Islamic duty -- an integral part of his jihad. Systemically, over the course of decades, he dispatched his top lieutenants to attempt to purchase or develop nuclear and biochemical WMD. He has never given up the goal; indeed, in a 2007 video, he repeated his promise to use massive weapons to upend the global status quo, destroy the capitalist hegemony, and help create an Islamic caliphate.
Since the mid-1990s, al Qaeda's WMD procurement efforts have been managed at the most senior levels, under rules of strict compartmentalization from lower levels of the organization, and with central control over possible targets and the timing of prospective attacks.
Graham Allison in Foreign Policy:
In 2007, Congress established a successor to the 9/11 Commission to focus on terrorism using weapons of mass destruction. This bipartisan Commission on the Prevention of WMD Proliferation and Terrorism issued its report to Congress and the Obama administration in December 2008. In the commission's unanimous judgment: "it is more likely than not that a weapon of mass destruction will be used in a terrorist attack somewhere in the world by the end of 2013."
Faced with the possibility of an American Hiroshima, many Americans are paralyzed by a combination of denial and fatalism. Either it hasn't happened, so it's not going to happen; or, if it is going to happen, there's nothing we can do to stop it. Both propositions are wrong. The countdown to a nuclear 9/11 can be stopped, but only by realistic recognition of the threat, a clear agenda for action, and relentless determination to pursue it.