Concerns are growing that Al Qaeda or a related group could detonate a "dirty bomb" that would spew radioactive fallout across an American or European city, according to intelligence analysts, diplomats and independent nuclear experts.
Although safeguards protecting nuclear weapons and their components have improved, experts said the radioactive materials that wrap around conventional explosives to create a contaminating bomb remained available worldwide -- and were often stored in non-secure locations.
Detonating a dirty bomb would not cause the death and devastation wrought by a nuclear weapon, but officials and counter-terrorism experts predicted that it would result in some fatalities, radiation sickness, mass panic and enormous economic damage.
Intelligence agencies have reported no reliable, specific threats involving dirty bombs or nuclear weapons, but senior U.S. and European officials and outside experts said several factors had heightened fears in recent weeks.
They said concerns were focused on three Al Qaeda operatives who led experiments involving dirty bombs and chemical weapons and on widely held suspicions that a special wing of the terrorist network was planning a spectacular attack.
They also said that chatter justifying the use of nuclear weapons against the U.S. had increased on radical Islamic websites as the occupation of Iraq stretches into its second year.
One focus of anxiety is the Athens Olympic Games in August. Recent security exercises there concentrated on mock attacks involving a dirty bomb, a chemical explosion and a hijacked jetliner.
Another potential target is the North Atlantic Treaty Organization summit scheduled for June in Istanbul, Turkey, which will be attended by President Bush. The threat was underlined by Turkey's disclosure Monday that it had arrested members of a group linked to Al Qaeda who reportedly planned to bomb the summit.
The threat of attack is great enough that a senior European intelligence official, speaking on condition of anonymity, said it is "not a matter of if there is a nuclear-related attack by Al Qaeda, but when it occurs."
The warning echoed remarks made last June by Eliza Manningham-Buller, director of Britain's domestic intelligence service, MI5. She said renegade scientists have aided Al Qaeda's efforts to develop chemical, biological, radiological and nuclear weapons, sometimes referred to as CBRN.
"Sadly, given the widespread proliferation of the technical knowledge to construct these weapons, it will only be a matter of time before a crude version of a CBRN attack is launched at a major Western city and only a matter of time before that crude version becomes something more sophisticated," she told a London think tank.
Experts inside and outside government said sophisticated extremists have the ability to plan and execute the detonation of a dirty bomb. They had no answer for why a dirty bomb has not been unleashed.
"I'm very surprised that a radiological device hasn't gone off," said Matthew Bunn, a nuclear expert at Harvard's Belfer Center for Science and International Affairs. "There is a bigger puzzle -- why no Al Qaeda attacks since Sept. 11 in the U.S.?"
The European intelligence official said planning for a large-scale attack has suffered setbacks with the arrests of numerous Al Qaeda operatives. But, he added, "the division is still focused on spectaculars, and they take three or four years to plan and execute."
U.S. intelligence has long known that Al Qaeda coveted a nuclear weapon, but there is no evidence that it has succeeded in getting one.
"We won't know if Al Qaeda has its hands on this kind of material until it is too late," said M.J. Gohel, head of the Asia-Pacific Foundation in London.
Building a dirty bomb is far easier, and the terrorist network's attempts to do so have been documented through evidence uncovered in Afghanistan and elsewhere.
Three men identified as Al Qaeda's weapons of mass destruction committee would likely plan the attack, said two European intelligence officials and independent experts.
The committee leader is Midhat Mursi, an Egyptian chemical engineer also known as Abu Khabab. Officials said he is regarded as Al Qaeda's master bomb builder and is one of the group's most-wanted fugitives -- although there have been unconfirmed reports that Mursi is in U.S. custody.
A second member is Assadalah Abdul Rahman, a son of Omar Abdul Rahman, the blind Egyptian cleric convicted in the 1993 bombing of the World Trade Center. The son ran a camp near Jalalabad, Afghanistan, that provided training in chemical weapons.
The third was identified as Abu Bashir Yemeni, who also worked in the Afghan training camps and at a house in Kabul, the Afghan capital, that authorities suspect was the committee's headquarters.
Documents describing research into chemical weapons and dirty bombs were discovered in the house by CNN in late 2001. In caves used by Al Qaeda in Afghanistan, U.S. troops also found detailed instructions on how to manufacture and deploy a dirty bomb.
Much of Al Qaeda's leadership has been destroyed, but counter-terrorism experts said the organization is divided into two tiers. The more visible wing is loosely aligned with other extremist groups and helps organize small-scale attacks on "soft targets," such as the conventional bombings in Bali, Indonesia; Casablanca, Morocco; Istanbul; and Madrid.
Long-term planning for a bigger attack in the U.S. or Europe is being carried out by a second core group of experienced Al Qaeda figures, including the weapons committee, according to the European intelligence official and two counter-terrorism experts.
"There is a sense that one part of Al Qaeda is waiting and putting into place the big, spectacular attack," said Magnus Ranstorp, director of the Center for the Study of Terrorism and Political Violence at the University of St. Andrews in Scotland. "It will come out of left field, and it may well be a dirty bomb."
U.S. authorities say they thwarted the beginnings of a dirty bomb plot with the arrest of Jose Padilla in Chicago in May 2002. Al Qaeda leaders had sent Padilla, a U.S. citizen, on a reconnaissance mission, authorities say. He is being held as an enemy combatant.
Although less devastating than a nuclear explosion, which could cause an astronomical number of deaths, a dirty bomb would have severe economic and psychological consequences, experts say.
A computer simulation by the Federation of American Scientists found that detonating a device containing 1.75 ounces of cesium in Lower Manhattan would distribute radioactive fallout over 60 square blocks.
Immediate casualties would be limited to people hurt by the blast, but the simulation suggested that there would be cases of radiation sickness and that relocation and cleanup costs would reach tens of billions of dollars.
Steven E. Koonin, a counter-terrorism consultant to the U.S. government and a former Caltech provost, said even small amounts of contamination would send hundreds of thousands of people to hospitals for screenings and could leave dozens of buildings uninhabitable under current radiation limits.
"There would be billions of dollars in economic damage," he said.
Terrorists could make a similar impact without an explosion. A diplomat in Vienna described the consequences of leaving an open container of cesium in a public place for a day or two before reporting the attack.
"Contamination would have spread across a wide area and people would be in an absolute panic," said the diplomat, who asked that his name not be used.
Fears of contamination, more than the actual danger, are why experts often describe dirty bombs as "weapons of mass disruption" rather than weapons of mass destruction.
Although authorities say trafficking in enriched uranium and plutonium for nuclear weapons has dropped in recent years, the number of reports involving radioactive materials suitable for dirty bombs has increased.
Most of the 60 incidents of trafficking in nuclear material reported to the International Atomic Energy Agency, the United Nations' nuclear watchdog, in 2003 involved radioactive, not fissile, material.
Two of the most serious cases involved cesium-137, an isotope used in radiotherapy. In its most common form, cesium is the perfect dirty bomb ingredient -- a fine, talc-like powder that is easily dispersed and binds to asphalt and concrete.
In June, U.S. and Thai authorities arrested a teacher in Bangkok when he tried to sell a small amount of cesium for $240,000. Thai police said the cesium originated in a former Soviet republic.
Although Russia and other former Soviet regions have improved security over nuclear weapons and fissile material, officials with international agencies said controls over radioactive material suitable for dirty bombs need improvement.
"Sensitive fissile material is well protected, but we have to make it more difficult for any terrorist organization to get these other materials," said Anita Birgitta Nilsson, head of nuclear security for the International Atomic Energy Agency.
Police in the Georgian capital, Tbilisi, discovered two metal containers of radioactive material and a small quantity of nerve gas in a taxi a year ago. Police said the material was destined for sale.
One container held strontium-90, a highly radioactive substance used during the Soviet era to power generators in remote areas. The other contained cesium, which had widespread medical and agricultural uses under the Soviets.
Georgian security and nuclear officials said there are no records of how much radioactive material remains scattered around the country. Safeguards are weak, they added.
"We can show you what we possess, but it's impossible to know what we don't possess," said Shukuri Abramidze, head of the Atomic Energy Commission of the Georgian Academy of Sciences.
Even when material is collected, there is no central storage site in Georgia. As a result, it is stored in makeshift facilities that authorities acknowledge offer meager protection.
One site is a concrete shed west of Tbilisi that officials said during a recent tour contained enough cesium, strontium and cobalt to contaminate Los Angeles.
An elderly, unarmed man guards the shed in exchange for free use of a nearby house. Its alarm system is often disabled because of power failures.
Officials from the IAEA and the U.S. Department of Energy installed steel doors and an underground container at the shed, but Georgian authorities worry that it is not enough.
"The danger is quite real that terrorists could take these sources and use them in dirty bombs," said Zaur Chankseliani, head of the radiological institute 100 yards away.
U.S. officials are concerned that the material is within reach of Islamic extremists and Chechen rebels in the Pankisi Gorge, a remote area near Georgia's border with the breakaway Russian republic of Chechnya.
Chechen rebels were responsible for the only known incident involving a dirty bomb. In 1995, they planted an explosive device containing cesium in a Moscow park, then informed reporters of its location before it was detonated. Authorities believe that the tipoff was a warning but that the Chechens were not prepared to risk the retaliation likely to be provoked by detonating a dirty bomb.
Extremists searching for the ingredients of a dirty bomb need not look as far as Georgia. Sources are plentiful throughout the world, including the United States, and often they are not stored securely, experts said. The IAEA estimates that 110 countries lack adequate controls over material that could be used in a dirty bomb.
Nearly 10 million containers of radioactive material -- including the detritus from medical facilities -- exist in the United States and 49 other countries, according to a 2003 survey by the congressional General Accounting Office.
The agency said that each year, hundreds of containers are lost or stolen in the U.S. and other countries, particularly in the former Soviet Union. The report warned that the radioactive material posed a "national security threat" and urged that controls be strengthened worldwide.