The gravest risks from al Qaeda combine its affinity for big targets and its announced desire for weapons of mass destruction.
Among all the upheavals of war with al Qaeda, the surest indicator of the historic stakes is the ongoing rotation of top U.S. government managers - scores at a time - into a bunker deep underground and far from Washington.
Leaders at the top of al Qaeda's hierarchy, the evidence shows, completed plans and obtained the materials required to manufacture two biological toxins - botulinum and salmonella - and the chemical poison cyanide.
Unsettling signs of al Qaeda's aims and skills in cyberspace have led some government experts to conclude that terrorists are at the threshold of using the Internet as a direct instrument of bloodshed.
Searches of al Qaeda sites in Afghanistan, undertaken since American-backed forces took control there, are not known to have turned up a significant cache of nuclear materials.