Nuclear weapons experts say the greatest threat posed by terrorist groups seeking nuclear weapons comes from their stealing a warhead or obtaining highly enriched uranium or plutonium from which they could fashion a nuclear device.

"We have been worrying about this kind of threat emerging for years," Roger L. Hagengruber, senior vice president for national security and arms control at Sandia National Laboratories, said Friday. Since the Sept. 11 attacks, "my worry index has gone up substantially," Hagengruber said, adding that the skills shown by the al Qaeda terrorist network putting together that operation demonstrate "the potential is there."

Hagengruber said the first threat of terrorists like Osama bin Laden, the al Qaeda leader, acquiring a nuclear capability comes from their stealing a weapon. That is "the most devastating scenario," according to the International Atomic Energy Agency (IAEA).

But Hagengruber noted that U.S. weapons have built-in locks to prevent their being exploded, a secure system that he said would take outside scientists years to break.

Hagengruber added that, having worked with the Russians on security for their weapons, "I just don't think Russians are missing weapons, they care about this . . . they care about safety and security about theirs as we do about ours."

Bin Laden or others obtaining highly enriched uranium is the second greatest threat, according to the IAEA and other experts.

IAEA Director General Mohamed ElBaradei said on Monday that "while we cannot exclude the possibility that terrorists could get hold of some nuclear material, it is highly unlikely they could use it to manufacture and successfully detonate a nuclear bomb."

Paul Leventhal, president of the Nuclear Control Institute, a research and advocacy center on nuclear proliferation and terrorism, disagreed. He said a team of five former U.S. weapons designers "found that terrorists indeed would be capable of making an effective, first-generation nuclear weapon if they could obtain enough reactor-grade plutonium or highly enriched uranium."

But those designers said terrorists working with the material would have to be trained in physical, chemical and metallurgical properties of nuclear materials, the characteristics of their fabrication, high explosives, chemical propellants, hydrodynamics and electrical circuitry.

"It is exceedingly unlikely that any single individual, even after years of assiduous preparation, could equip himself to proceed confidently in each part of the diverse range of necessary knowledge and skills," the panel wrote in a 1997 paper. It concluded that at least three specialists would be required.

Hagengruber said that if an aspiring bomb builder had enough pure, highly enriched uranium, and had some fundamental understanding of nuclear weapons design, he "could create a situation with a 10 percent chance of having a sizable explosive yield."

But obtaining the roughly 30 kilograms -- or 65 pounds -- of highly enriched uranium required for such a result is a difficult task, according to counterterrorism experts.

Much less plutonium is needed for a nuclear explosion, but it is far more dangerous to handle and much more difficult to treat in a manner that would cause a nuclear explosion.

If a terrorist group succeeded in obtaining enough fissile material, it would need a place where it could work "uninterrupted for a significant period of time," according to David Albright and his colleagues at the Institute for Science and Security. "The necessary weaponization facilities can be small," Albright wrote in September, noting that South Africa's "initial nuclear weapons effort in the 1970s used small, rudimentary facilities that were extremely difficult to detect by overseas intelligence agencies."

One other consideration is what is known as a dirty bomb, a device containing radioactive materials and explosive chemicals that is detonated to contaminate a selected area.

The potential impact from such a device can be measured using the experience recorded in 1987 in the Brazilian city of Goiania. There, some scrap scavengers broke into an abandoned radiological clinic and stole a capsule containing a little more than an ounce of highly radioactive cesium 137. The capsule was cut into more than 100 pieces, which were passed along to family members and friends around the city.

"Fourteen people were overexposed to radiation out of 249 contaminated," according to the IAEA. "Four subsequently died and more than 110,000 had to be continuously monitored. To decontaminate the area, 125,000 drums and 1,470 boxes were filled with contaminated clothing, furniture, dirt and other materials; 85 houses had to be destroyed."