Stephen Younger, director of the Defense Threat Reduction Agency, said extensive searches in Afghanistan showed al Qaeda was interested in nuclear technologies, as well as biological and chemical weapons.
He said they had made little progress toward building their own bombs before U.S. forces intervened last fall, drove the Taliban regime from power and sent surviving al Qaeda leaders into hiding.
"Al Qaeda has been trying to get a weapons of mass destruction capability," he told a group of reporters Wednesday. "I think they had a limited infrastructure in Afghanistan to produce it indigenously.
"However, that doesn't mean that they don't have a different capability elsewhere," he added. Later he said this meant that al Qaeda leaders may have connections in other countries that already have the technological base for building nuclear weapons. They have the money to make such links, he said, and they have "access to people in countries with advanced technological capability."
Defense Secretary Donald H. Rumsfeld has publicly raised the possibility that Iraq could be such a supplier for al Qaeda or other international terrorist groups.
Al Qaeda's interest in biological weapons seemed to be focused mainly on anthrax, Younger said.
In light of the Sept. 11 attacks and concerns within the Bush administration that international terrorists might link up with Iraq to obtain weapons of mass destruction, the Pentagon is exploring new ways to neutralize or destroy biological and chemical weapons that might be stored underground.
Younger said one possibility is a warhead that would encapsulate a biological or chemical weapons facility with a hard or sticky foam rather than blow it up with conventional bombs.
Another possibility is a nonexploding warhead that spreads flammable materials to incinerate biological agents.
Both approaches are still on the drawing board. They would be alternatives to conventional high-explosive warheads, which might allow contaminants to escape, threatening civilians or U.S. troops.
"It's not as simple as blowing it up," Younger said.
Younger said that although the United States does not know what kinds of weapons Iraq may have developed since U.N. inspections ended in 1998, it is a "reasonable assumption" based on Saddam Hussein's track record that the Iraqi president either has or is pursuing weapons of mass destruction.
Iraq claims it has no weapons of mass destruction.
The Pentagon is contemplating other unpleasant scenarios that could emerge in Iraq or elsewhere, Younger said.
One possibility: a U.S. satellite detects a Scud ballistic missile, possibly armed with biological agents, being readied for launch. What could the United States do to stop it if there were no U.S. strike aircraft nearby and ready?
In the future, an answer might be to strike with a non-nuclear intercontinental ballistic missile, which has the advantage of very high speed. For now, all the United States' ICBMs on land and at sea are armed with nuclear warheads. To switch some to non-nuclear roles would create political issues; launching one in a crisis would raise fears in Moscow and elsewhere that a nuclear war was under way.
Younger's agency also is working on other kinds of advanced non-nuclear weapons. He said experiments have been done on arming a Hellfire air-to-ground missile with a thermobaric warhead, which ignites an explosive mist that sends a powerful shock wave through a cave or tunnel, annihilating everything and everyone inside.
Such a weapon is likely to be ready for use "in fairly short order," Younger said without being more specific.
At least one thermobaric weapon was used by the Air Force in Afghanistan, but it has never been developed in a warhead small enough to fit onto a Hellfire missile. Although the Hellfire normally is launched from a helicopter, some have been fired in Afghanistan from Predator unmanned drones.