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. The modus operandi has been top-down -- more similar to the 9/11 attacks than to more recent bottom-up efforts, like the attempted bombing of Flight 253. For instance, al Qaeda deputy chief Ayman al-Zawahiri personally shepherded the group's ultimately unsuccessful efforts to set off an anthrax attack in the United States.
Al Qaeda concentrated its efforts on nuclear devices in the run-up to the September 11, 2001 attacks. Based on the timing and nature of its WMD-related activity in the 1990s, al Qaeda hoped to use such weapons in the United States during an intensified campaign following the 9/11 attacks. There is no indication that the fundamental objectives that lie behind its WMD intent have changed over time.
Al Qaeda seems to have failed in its mission to successfully detonate WMD due to its overpowering interest in such big-casualty, big-impression attacks. The organization has not pursued simpler, cheaper, and easier-to-use technologies, like crude toxins and poisons, with anything like the same fervor. To be sure, experimentation with and training in such agents was standard fare in al Qaeda's camps in Afghanistan before 9/11. But bin Laden and his top associates left the initiative to lower-ranking planners and individual cells. Once, Zawahiri even canceled a planned attack on the New York City subway in lieu of "something better" that never materialized.
But just because "something better" has never materialized, and just because the threat of WMD terrorism has been used to political ends, does not mean that WMD are not a threat. This chronology provides the knowable extent of al Qaeda's interest in, plans to obtain, and efforts to use the world's most deadly weapons.