Washington ? Members of Congress have warned about the dangers of suitcase nuclear weapons. Hollywood has made television shows and movies about them. Even the Federal Emergency Management Agency has alerted Americans to a threat – information the White House includes on its Web site.

But government experts and intelligence officials say such a threat gets vastly more attention than it deserves. These officials said a true suitcase nuke would be highly complex to produce, require significant upkeep and cost a small fortune.

Counterproliferation authorities do not completely rule out the possibility that these portable devices once existed. But they do not think the threat remains.

“The suitcase nuke is an exciting topic that really lends itself to movies,” said Vahid Majidi, the assistant director of the FBI’s Weapons of Mass Destruction Directorate. “No one has been able to truly identify the existence of these devices.”

Majidi and other government officials say the real threat is from a terrorist who does not care about the size of his nuclear detonation and is willing to improvise, using a less deadly and sophisticated device assembled from stolen or black market nuclear material.

Yet Hollywood has seized on the threat. For example, the Fox thriller “24” devoted its entire last season to Jack Bauer’s hunt for suitcase nukes in Los Angeles.

Government officials have played up the threat, too.

In a FEMA guide on terrorist disasters that is posted in part on the White House’s Web site, the agency warns that terrorists’ use of a nuclear weapon would “probably be limited to a single smaller ‘suitcase’ weapon.”

“The strength of such a weapon would be in the range of the bombs used during World War II. The nature of the effects would be the same as a weapon delivered by an intercontinental missile, but the area and severity of the effects would be significantly more limited,” the paper says.

The genie that escaped

During the 1960s, intelligence agencies received reports from defectors that Soviet military intelligence officers were carrying portable nuclear devices in suitcases.

The threat was too scary to stay secret, government officials said, and word leaked out. The genie was never put back in the bottle.

But current and former government officials who have not spoken out publicly on the subject acknowledge that no U.S. officials have seen a Soviet-made suitcase nuke.

The idea of portable nuclear devices was not a new one.

In the 1950s and 1960s, the U.S. made the first ones, known as the Special Atomic Demolition Munition. It was a “backpack nuke” that could be used to blow up dams, tunnels or bridges. While one person could lug it on his back, it had to be placed by a two-man team.

These devices never were used and now exist – minus their explosive components – only in a museum.

Following the U.S. lead, the Soviets are believed to have made similar nuclear devices.

’60 Minutes’ report

Suitcase nukes have been a separate problem. They attracted considerable public attention in 1997, thanks to a “60 Minutes” interview and other public statements from retired Gen. Alexander Lebed, once Russia’s national security chief.

Lebed said the separatist government in Chechnya had portable nuclear devices, which led him to create a commission to get to the bottom of the Chechen arsenal, according to a Center for Nonproliferation Studies report. He said that when he ran the security service, the commission could find only 48 of 132 devices.

The numbers varied as he changed his story – sometimes he stated that 100 or more were missing. The Russians denied he was ever accurate.

Even more details emerged in the summer of 1998, when former Russian military intelligence officer Stanislav Lunev – a defector in the U.S. witness protection program – wrote in his book that Russian agents were hiding suitcase nukes around the U.S. for use in a possible future conflict.

“I had very clear instructions: These dead-drop positions would need to be for all types of weapons, including nuclear weapons,” Lunev testified during a congressional hearing in California in 2000, according to a Los Angeles Times account.

Naysayers noted that he was never able to pinpoint any specific location.

In a 2004 interview with the Kremlin’s Federal News Service, Colonel-General Viktor Yesin, former head of the Russian strategic rocket troops, said he believes that Lebed’s commission may have been misled by mock-ups of special mines used during training.

Yesin believed that a true suitcase nuke would be too expensive for most countries to produce and would not last more than several months because the nuclear core would decompose so quickly. “Nobody at the present stage seeks to develop such devices,” he asserted.

Some members of Congress remained convinced that the suitcase nuke problem persists. Perhaps chief among these lawmakers was Curt Weldon, a GOP representative from Pennsylvania who lost his seat in 2006.

The science

Majidi joined the FBI after leading Los Alamos National Laboratory’s prestigious chemistry division. He uses science to make the case that suitcase nukes are not a top concern.

First, he defines what a Hollywood-esque suitcase nuke would look like: a case about 24 inches by 10 inches by 12 inches, weighing less than 50 pounds, that one person could carry. It would contain a device that could cause a devastating blast.

Nuclear devices are either plutonium, which comes from reprocessing the nuclear material from reactors, or uranium, which comes from gradually enriching that naturally found element.

Majidi says it would take about 22 pounds of plutonium or 130 pounds of uranium to create a nuclear detonation. Both would require explosives to set off the blast, but significantly more for the uranium.

Although uranium is considered easier for terrorists to obtain, it would be too heavy for one person to lug in a suitcase.

Plutonium would require the cooperation of a state with a plutonium reprocessing program. It seems highly unlikely that a country would knowingly cooperate with terrorists because the device would bear the chemical fingerprints of that government. “I don’t think any nation is willing to participate in this type of activity,” Majidi said.

That means the fissile material probably would have to be stolen. “It is very difficult for that much material to walk away,” he added.

There is one more wrinkle: Nuclear devices require a lot of maintenance because the material that makes them so deadly also can wreak havoc on their electrical systems.

“The more compact the devices are – guess what? – the more frequently they need to be maintained. Everything is compactly designed around that radiation source, which damages everything over a period of time,” Majidi said.