"At first we were just shocked," Maj. Leri Omiadze, the ranking officer at the scene, recalled later. "Then we all started backing away slowly."

Inside Makeria's boxes were two capsules of highly radioactive metals -- strontium and cesium -- of a type that terrorism experts say can be used in a dirty bomb, a device that spews radiation but does not trigger a nuclear explosion. A third container held a vial of brown liquid that Georgian police identified as the substance used in mustard gas, one of the earliest chemical weapons. Only later did police learn Makeria's role in the affair. He was a courier for criminals trading in components and materials for weapons of mass destruction.

In a scheme still not fully understood, the boxes were delivered to Makeria by another Georgian, a man with a history of drug offenses. Makeria's job was to carry the boxes by train from Tbilisi to Adzharia province, a troubled enclave on Georgia's southwestern frontier. From there, police believe, they were to be transported by other couriers across the border into Turkey or perhaps even Iran, for delivery to an expectant customer. The buyer's identity remains unknown.

What is certain is that the Georgians who sought to profit from selling components of a dirty bomb are far from unique.

There have been dozens of cases of trafficking in radiological materials over the past three years, along with what some weapons experts describe as a disturbing new trend. While most sellers of such materials have traditionally been amateurs -- opportunists and lone actors in search of easy profits -- authorities are now seeing a surge of interest among criminal groups. In a string of incidents from the Caucasus and Eastern Europe to West Africa and South America, gangs have stalked and stolen radiological devices to sell for profit or to use in crimes ranging from extortion to murder.

The new interest in radiological material by smugglers and criminal networks complicates an already difficult task confronting governments: how to stop terrorists from obtaining any of the tens of thousands of powerful radiological sources around the world that are currently in private hands or have simply been discarded. In Georgia and other unstable corners of the world, radioactive materials are turning up on black markets alongside more traditional contraband, such as drugs or Kalashnikov rifles.

They are a currency of the global gray zone, a dangerous mixture of failed states, porous borders and weak law enforcement, where the tools of terrorism are bought and sold.

The involvement of professional smugglers and criminals only increases the odds that some of the radiological materials will end up in the hands of terrorists, U.S. experts say. Already, the sheer volume of such materials in circulation has prompted scientists at the Los Alamos National Laboratory to conclude, in a study released in September, that a dirty bomb "attack somewhere in the world is overdue."

So serious is the threat that both the Bush administration and the International Atomic Energy Agency have launched major initiatives within the past 18 months to find and lock up abandoned radiological material across the globe. At the Energy Department, Secretary Spencer Abraham (news - web sites) has made preventing a dirty-bomb threat a top priority, on a par with long-established programs to secure nuclear stockpiles in the former Soviet Union.

A dirty bomb, or "radiological dispersion device" in the jargon of defense experts, is not a nuclear weapon but rather a crude device that uses conventional explosives or other means to spread radiation over a wide area. Compared to true nuclear weapons or even to biological or chemical weapons, they are technologically simple, and well within the grasp of international terrorist groups, nuclear experts say.

Documents seized from training camps in Afghanistan (news - web sites) two years ago by U.S. forces showed that al Qaeda leaders there planned to build a dirty bomb and may have begun gathering materials for one. Iraq (news - web sites), which struggled in vain for a decade to master the complexities of a nuclear weapon, built and tested a dirty bomb in the 1980s before abandoning the program on the grounds that it was ineffective against military targets, according to U.N. weapons inspectors.

Such a bomb would likely unleash panic and trigger economic and social upheavals. Even a moderately sized dirty bomb exploded in a modern city could contaminate large swaths of real estate with radiation, rendering some areas uninhabitable for months or years.

Last year, the Federation of American Scientists conducted a computer simulation to determine the impact of exploding less than two ounces of cesium-137, about 3,500 curies, in the heart of Manhattan. (A curie is a unit used to measure radioactivity. Experts say that a device of only a few dozen curies could make an effective bomb.) In the simulation, fine cesium particles spread across an area covering 60 square blocks. Cleanup and relocation following the blast would take years to complete and cost tens of billions of dollars, the study found.

Whether the radiation from such a blast would cause deaths or injuries is a subject of renewed debate. A view long held by radiation experts was that the human toll would be minimal; any deaths and injuries would be those caused by the blast effects of the explosion itself.

Now scientists aren't so sure. A new analysis, drawn from medical studies of radiation accidents, sees a significant health threat in the clouds of radioactive dust thrown up by a dirty-bomb explosion. The diluted radioactivity in those dust clouds would probably be too weak to cause serious harm. But, according to a new National Defense University analysis expected to be released next month, people near the blast site could suffer serious internal injuries from highly radioactive particles that enter the body through the nose and mouth and lodge in sensitive tissues. The severity of the injuries would depend on the type of radioactive material used, how it is spread, and how quickly the victims can be treated.

"If the particles are in a respirable form, they can do considerable damage -- to the lungs, to the digestive system, to the immune system," said Peter Zimmerman, chairman of the panel that produced the study. "Overall, the effects could be much worse than many of us previously thought."

Dozens of smuggling routes for nuclear and radiological materials have been charted over the past decade, but since 1999 a clear favorite has emerged. Judging from cases reported to police, nuclear traffickers have discovered abundant opportunity in Europe's southeastern flank: the Black Sea and Caucasus states that have long served as a crossroads linking Europe, the Middle East and Asia.

Topping the list is Georgia, the former Soviet republic where huge crowds of demonstrators recently forced President Eduard Shevardnadze to resign. The small nation of 5 million suffers from porous borders, official corruption and rampant smuggling, problems exacerbated by three ethnic rebellions -- in the provinces of Abkhazia and South Ossetia in the north, and Adzharia in the south -- and regular incursions by guerrillas in the eastern region bordering Chechnya (news - web sites). In the conflict zones, trafficking in contraband has gone from a sideline trade to a thriving industry that supports tens of thousands of people, including, by some accounts, leaders of the rebel movements.

"Today, it's smuggling that keeps the separatist movements alive," said Aleko Kupatadze, a black-market specialist at the Transnational Crime and Corruption Center in Tbilisi. "Many of the guerrillas are really professional criminals who sometimes even switch sides. The violence you see has less to do with ethnic conflict than with disagreements over how the spoils are divided."

Radioactive materials are now caught up in the illicit trade. Georgia has been a dumping ground for Soviet-era radioactive hardware and waste, some of it extraordinarily lethal. Abandoned radioactive devices are found regularly in Georgia's rugged hills, often after a villager turns up with severe radiation burns. Two years ago this month, three woodcutters in northern Georgia nearly died of radiation injuries after stumbling across a Soviet-built generator powered by strontium with a radioactivity level of 40,000 curies. Nine such devices have been found in Georgia since the mid-1990s, and as many as three more are feared to be still missing.

"We inherited chaos. Radiological equipment has turned up in garbage dumps, even in sewage," said Dato Bakradze, director of international security and conflict management for Georgia's National Security Council. The possibility that Georgian terrorists or separatists might obtain one of the devices, he said, poses a "direct physical danger to our own country."

Since the early 1990s, Georgian police have been intercepting radioactive flotsam from amateur sellers hoping to profit from their discoveries. Lately, the materials offered for sale have become more sophisticated, and so have the traffickers.

At least three times since 1999, officials have discovered kilogram-quantity caches of uranium in vehicles leaving or entering Georgia. In the most recent case, on June 26, just over a pound of uranium was seized at the Georgia-Armenia border by guards armed with U.S.-supplied radiation detectors, according to Georgian security officials. Tests to determine the origin and enrichment level of the uranium were carried out with the help of U.S. Energy Department officials. The agency has declined to release the results. Georgian officials say they believe the material originated in Russia and was being transported through Georgia for resale in Iran.

The smuggling incident uncovered on May 31 in Georgia's capital appears to have been bolder still. If the plan had unfolded as intended, the radioactive materials would have moved by public train through the heart of the country's most populous city, into the troubled Adzharia province, a center of ethnic clashes and long-simmering hostility toward Georgia's central government.

Makeria, 33, the taxi driver, has told police he knows almost nothing about the origins or destination of the deadly cargo. In fact, he may not have realized the contents were radioactive, despite warning labels written in English and Russian, said Tamaz Alania, chief of the criminal division of Georgia's Internal Affairs Ministry.

"It is at least possible that Makeria did not know," Alania said in an interview. "He seemed confused and nervous when we first questioned him. And when we explained what was in the boxes he became much more nervous."

Makeria told police he picked up the unusually heavy green cartons from Giorgi Samkhakiuli, 29, an acquaintance of his father-in-law, who asked him to keep the boxes at his home in Adzharia until someone else came to pick them up. But Samkhakiuli, a man described by police as having a history of drug offenses, vanished after the smuggling plot was foiled. Investigators continue to pursue leads, but the search for others appears to have stalled.

Police have learned that the larger of the two radiological elements, a capsule of powdery cesium, was manufactured in the Soviet Union in the 1970s for industrial use. While the cesium has lost more than half of its original potency, it still contains enough radioactivity to seriously injure or kill, investigators said. Police were baffled about the possible origin of the mustard gas substance, which was still being analyzed.

Where and how the smuggled materials were to be used, police can only guess. But those responsible went out of their way to collect and package three radioactive materials with no known uses other than to terrorize or kill, said Malkhaz Salakaia, the investigations director at Georgia's Ministry of State Security.

"At this point we have to assume there are other people behind Samkhakiuli," Salakaia said. "And we cannot exclude that a criminal act was envisioned."

The radiological materials coveted by criminal groups are not found only in former Soviet states. Tens of thousands of powerful radioactive devices are currently in use across a wide range of industries, from medicine to metallurgy to mining. Some of them, because of their size, potency and availability, have become popular targets for thieves -- and a nightmare for counterterrorism experts.

One such device is known as a "well-logger," an instrument used by energy companies and geologists to search for underground oil fields. In well-logging, a powerful capsule of radioactive metal -- usually americium, iridium or strontium -- is lowered into a well shaft to probe for oil deposits, using beams of neutron and gamma radiation that penetrate dense rock. Then the radiation is measured to look for evidence of oil beneath the rocks. When not in use, the core is kept in a shielded canister the size of a small beer keg.

Well-loggers don't pack enormous amounts of radiation. But what they carry is dangerous.

"It's a neutron source," said Abel Gonzales, the radiation safety chief for the International Atomic Energy Agency, the U.N. nuclear watchdog agency, referring to the type of deep-penetrating, tissue-destroying radiation emitted by well-loggers. If a dirty bomb is the objective, he said, "you could make something very nasty with that."

They also are easy to obtain. Tens of thousands of well-loggers are currently in use around the world, often in remote areas where they are liable to be stolen or lost.

One particularly worrisome criminal plot that recently came to light involved the theft of five iridium devices in Ecuador by a criminal gang that demanded -- and received -- thousands of dollars in payments for their return. It was the first known case of successful blackmail involving radiological material, and U.S. and U.N. experts fear the pattern could be repeated.

In a carefully planned, nighttime burglary Dec. 9, thieves broke into a storage shed in Quininde, in the coastal province of Esmeraldas, to steal the devices, which were owned by the firm Interinspec, according to accounts by investigators at the U.N. nuclear watchdog agency and Ecuador's Atomic Energy Commission. One of the thieves knew precisely where the instruments were kept and how much they were worth. He was a former employee who had been recently fired in a job dispute, Ecuadoran officials said.

Within days, the company received a ransom demand, and despite protests from government investigators, it decided to pay. Officials familiar with the case say the firm's top manager agreed to a price of $1,000 for each of the five devices. "He thought this was the best way to take control of the five lost sources," said Marco Bravo Salvador, technical director of the Ecuadoran commission.

The thieves, however, returned only three of the well-loggers, apparently deciding to keep the other two. Meanwhile, the company lost a sixth source in January when it fell from a boat into Ecuador's Quininde River. A seventh device went missing when a work crew accidentally left it behind after finishing a project in a remote jungle location.

After a massive search involving hundreds of army troops, the sources lost in the jungle and river were recovered. The two others, presumably still in the hands of bandits, remain unaccounted for.

Another recent theft, viewed by U.S. and U.N. officials as especially grave, occurred in December when a large well-logger was stolen from a truck in Nigeria. The owner of the device was Halliburton Co., based in Houston, which conducted its own search for several weeks before notifying the U.N. nuclear watchdog of the loss.

The device reportedly was stolen while being hauled through the oil-rich Niger Delta, between the cities of Warri and Port Harcourt. Initially, the truck driver told police that someone swiped the instrument from his vehicle when he stopped at a roadside motel for a nap. Later, investigators began to find discrepancies in the driver's story.

"The hotel story didn't check out," said one official involved in the investigation, who spoke on condition that his name not be used. "The suspicion now is that the driver took it," apparently as part of a plot involving accomplices. The thief was apparently knowledgeable about such well-loggers because, out of several devices on the truck, he singled out the most powerful one, the official said.

Months of searches using radiation detectors turned up no trace of the missing well-logger. Then, two months ago, investigators got a break. A well-logger discovered in a private scrap yard in September turned out to be the same one that was stolen nine months earlier. The scrap yard was in Germany, more than 3,200 miles from the Niger Delta.