Usama bin Laden has said acquiring weapons of mass destruction — poison gas, killer diseases, radiation bombs and nuclear devices — is "a religious duty" and that all Americans are targets.

He has tried to get such weapons, say U.S. intelligence officials, who won't discuss whether the world's most wanted accused terrorist has succeeded.

"Does (bin Laden) have a nuclear bomb? I'd say no," said Sen. Richard Shelby, R-Ala., vice chairman of the Senate Intelligence Committee, who is privy to secret briefings from the CIA and other U.S. intelligence agencies. "We have to be aware of the possibility that he could launch a chemical attack or biological attack at any time. We need to prepare for it."

Since the Sept. 11 attacks on the World Trade Center and Pentagon, lawmakers and terrorism experts say they are undergoing a fundamental shift in thinking about doomsday weapons.

Now, it's clear that whoever conducted the attacks — and the exiled Saudi bin Laden remains the chief suspect — would not hesitate to use one of these nightmare devices if given the opportunity.

But don't panic, say the officials and experts.

"Bad guys are capable of developing these agents, but they are not very capable of delivering them," said Rep. Jane Harman, D-Calif., a member of the House Permanent Select Committee on Intelligence. "What is new is the wanton destruction of September 11 makes clear there would be no hesitation against using them."

Shelby said bin Laden has engineers and chemists among his ranks.

CIA Director George Tenet said both bin Laden and the Palestinian group Hamas are trying to obtain chemical weapons.

"Although terrorists we've pre-empted still appear to be relying on conventional weapons, we know that a number of these groups are seeking chemical, biological, radiological or nuclear agents," Tenet told the Senate Intelligence Committee last year. "We are aware of several instances in which terrorists have contemplated using these materials."

In addition, Tenet said, bin Laden's "operatives have trained to conduct attacks with toxic chemicals or biological toxins."

Witnesses in the 1998 embassy bombings trial said bin Laden, who has been indicted in the case, had sent people to Khartoum, the capital of Sudan, to buy South African uranium for $1.5 million. It is unclear from the trial transcript whether the purchase was made.

In addition, there have been reports of chemical and biological weapons being tested in his training camps in Afghanistan. A Taliban security officer, who accompanied bin Laden to a camp in eastern Kunar province, said he saw a North Korean training the militants in chemical weapons. It wasn't possible to independently confirm his report.

Fears of such an attack were heightened after investigators learned that one of the suspected Sept. 11 hijackers, Mohamed Atta, repeatedly tried to learn about crop-dusters in Florida. Some have suggested that terrorists would try to use the low-flying planes to spread a chemical or biological weapon over a populated area; fears of such plans caused the Federal Aviation Administration to keep crop dusters grounded nationwide last weekend.

Vince Cannistraro, the CIA's former counterterrorism chief, doesn't believe bin Laden has any usable weapons.

"If he had them in his inventory and could use them, he would use them," Cannistraro said. "There are no restraints."

All these weapons are hard to get and use effectively. Killing more than a few people with them takes technical skills, manufacturing and storage equipment and the means to smuggle them to targets.

Simply spraying a few gallons of poison vapor out of a crop duster won't necessarily kill a cityful of people, said Gary Ackerman, a terrorism expert at the Center for Nonproliferation Studies. A terrorist would have to account for numerous factors, such as wind speed, altitude and nozzle settings on the plane, to disperse the weapon effectively.

"You might kill a few people with that, but to cause a true mass casualty event — it's not that easy," Ackerman said.

Chemical agents such as sarin are considered the most likely means of a terrorist attack. Biological and radiological attacks are deemed less likely. Experts say an attack with a nuclear weapon, such as the much-feared "suitcase bomb," is considered highly unlikely.

One terrorist group, the Japanese cult Aum Shirikyo, tried to use a nerve agent in Tokyo in 1995. Their method was to poke holes in small bags of sarin left in subway terminals. While thousands were injured by the vapors, only twelve people were killed.

To former CIA officer and House Intelligence Committee Chairman Porter Goss, R-Fla., the most fearful weapon is a rapidly spreading disease, because it can affect several cities and threaten civil order.

U.S. cities, states and the federal government have trained firefighters, National Guard teams and other officials to respond quickly should one of these weapons be used.