The radiological weapon that experts fear most from terrorists isn't the multi-megaton ICBMs or even Hiroshima-sized bombs dropped from bombers.

The No. 1 most feared atomic weapon is a kind of bootleg nuke that could be made from a little bit of relatively low-grade nuclear material and a few sticks of dynamite.

Both the United States and the Soviet Union have created sophisticated nuclear bombs the size of suitcases, but the so-called "dirty bomb" wouldn't create the metal-melting nuclear blast those would. Instead, such a radiological bomb would kill more with its plume of toxic radiation.

"Had the terrorists at the World Trade Center used a radiological dispersal device, most parts of lower Manhattan would have been rendered uninhabitable," Tariq Rauf, director of the nonproliferation program at the Monterey Institute of International Studies, said.

The biggest appeal of such a weapon would be the fact that it requires neither a knowledge of physics nor the smuggling of weapons-grade uranium or plutonium.

"It's not that hard to build a radiological bomb since all you have to do is disperse a bunch of radioactive material," Brookings Institution senior fellow Michael O'Hanlon said.

Highly radioactive material is stored at over 1,000 facilities in 50 countries, according to the Carnegie Endowment for International Peace. The group says some facilities wouldn't be able to fend off would-be thieves looking for bomb ingredients.

To prevent nuclear smuggling, America relies on pressuring countries to safeguard weapons-usable and radioactive material. The U.S. hopes its border defenses and those of countries on likely transit routes would be enough to stop the material from making it into the hands of the wrong people.

But however remote officials say it is, nuclear terrorism remains a serious enough threat for President Bush to describe it in a speech to eastern European leaders on Tuesday. Court documents show that Usama bin Laden's Al Qaeda network has sought nuclear material. It is unclear whether the group succeeded.

"The probability is not zero," Tim Brown, an intelligence and military analyst with GlobalSecurity.org, said. "It's somewhere between zero and low."

Analysts who have examined the nuclear threat describe three separate scenarios.

— In the first, a so-called "suitcase nuke," probably from the ex-Soviet Union, could be sold to terrorists, who would seek to smuggle it into the United States, or within range of an U.S. overseas interest.

In the depths of the Cold War, the United States and Soviet Union each produced a few hundred portable nuclear weapons, Rauf said. The U.S. munitions were intended to slow a hypothetical Soviet invasion of western Europe by demolishing bridges and railways, he said.

Since the demise of the Soviet Union, rumors pointing to missing portable Soviet nuclear weapons have percolated through the defense community. None have been verified.

One stems from 1997 statements by Russian Gen. Alexander Lebed, who said some portable Soviet weapons were unaccounted for. Another originates in Russian press reports that Chechen rebels stole, or attempted to steal, small nuclear weapons from a military base.

In a third case, a pair of ethnic Russians were arrested in Miami in 1997 after offering to sell a suitcase nuke to undercover U.S. Customs agents. No evidence indicated the men had access to such a weapon, Customs spokesman Dean Boyd said.

"I'm not overly concerned about the suitcase-bomb threat," said Jon Wolfsthal, an associate with the Carnegie Endowment for International Peace. "The U.S. intelligence services have very high confidence that Russia has accounted for all its nuclear weapons."

— The second threat scenario involves a terrorist group building its own nuclear bomb using smuggled nuclear material.

The International Atomic Energy Agency has documented 18 cases of weapons-grade nuclear smuggling since 1993, among hundreds of cases of trafficking in radioactive materials. None of the cases involved enough for a bomb.

About a dozen countries have the material, but the largest amount — some 1,300 metric tons of plutonium and highly enriched uranium — sits in Russian weapons facilities and laboratories, according to the U.S. Department of Energy.

"It's very hard to track," Wolfsthal said. "There's no way to verify that materials aren't already missing. The Russians themselves don't know themselves how much they have."

Since 1992, U.S. agencies have spent more than $5 billion helping Russia upgrade security at the sites, and making sure weapons scientists were peaceably employed.

Border guards in the region trained by U.S. Customs have already seized radioactive materials, including, in 1999, 10 grams of weapons-grade uranium hidden inside a car traveling into Bulgaria.

Still, a terrorist-made A-bomb is a low-probability threat.

"Even Saddam Hussein's weapons program, after 10 years and several billion dollars in investments, was not able to make a nuclear bomb," Rauf said.

— The radiological bomb is a much simpler matter.

Depending on its potency, a contamination-spewing radiological bomb could kill dozens, hundreds, possibly thousands. Its toxic plume could render a square mile or more uninhabitable for a decade or longer. It would cause a huge cleanup and demoralize a city, perhaps a nation.

In the case of the 1986 Chernobyl nuclear reactor meltdown, a six-mile belt around the reactor is still uninhabitable, Rauf said.

"To a terrorist who is trying to create widespread panic, this option is more appealing," Rauf said. "You can see the white powder of anthrax, but not radiation. It can be carried by wind, by the water. In the public mind, a radiological device is more terrorizing."

The Associated Press contributed to this report