After the warehouse raid in northern Jordan, the word from authorities horrified the people of Amman.

Terrorists linked to Al Qaeda had assembled a fearsome array of chemicals and planned a bombing that would send a 2-mile-wide "poison cloud" over this Middle Eastern capital, killing as many as 80,000 people, military prosecutors said.

Usama bin Laden's foot soldiers had finally concocted a weapon of mass destruction.

• This is Part I of a three-part series. Click here to read Part II.

A year later, in the hard light of scientific scrutiny, that sinister scenario looks more fictional than factual.

"Eighty thousand! That would have been like Hiroshima. And that was an atomic bomb," says Samih Khreis, one of the alleged plotters' lawyers.

The defense attorneys aren't alone in scoffing at the "WMD" claim. International experts checking the suspects' supposed list of chemicals — from the industrial compound ammonium (search) to the explosive nitroglycerin (search) — say either the defendants or the Jordanian authorities, or both, had little inkling about the makings of a chemical weapon.

The compounds "may generate some toxic byproducts, but they're unlikely to result in significant deaths by poisoning," said Ron G. Manley (search) of Britain, a former senior U.N. adviser on chemical weapons.

The poison cloud of Amman is one more dubious episode in the story of the terrorist quest for doomsday arms, a dark vision that has become an axiom of today's counterterrorist strategy.

Four years into the "global war on terror," half the Americans surveyed this summer said they worry "a lot" about the possibility of such a WMD attack, according to the U.S. polling firm Public Agenda.

Concerns emerged in the 1990s when the Soviet Union's collapse left nuclear and other arms vulnerable to theft. Worries grew as "recipes" for mass-casualty weapons flashed around the Internet.

In 1998, Al Qaeda leader bin Laden told Time magazine that acquiring such arms to defend Muslims "is a religious duty." Three years later in Afghanistan, the U.S. military found Al Qaeda documents, crude equipment and other evidence of chemical and biological experimentation.

Al Qaeda's intent is clear, says a key U.S. intelligence analyst.

"The intent is there and you can see it in the 'fatwas' justifying the use" of WMD, Donald Van Duyn (search) of the FBI's Counterterrorism Division said in a Washington interview.

One fatwa, or Muslim religious decree, issued by radical Saudi cleric Nasser al-Fahd in 2003 at bin Laden's request, "authorized" the use of ultimate weapons "if the infidels can be repelled from the Muslims only by using such weapons."

"It may be only a matter of time before Al Qaeda or another group attempts to use chemical, biological, radiological or nuclear weapons," CIA Director Porter Goss (search) advised U.S. senators earlier this year.

Amid all the warnings, boasts and chilling tales, however, the daunting difficulties of fielding such weapons usually go unmentioned — along with Al Qaeda's glaring lack of expertise and stable home base, the unreliability of Internet "formulas," and the progress made worldwide in locking down the raw materials of the most destructive arms.

Amman's is one of many stories of exaggerated threats or ill-conceived plans. Others include:

— British police last year arrested eight people on suspicion of plotting a bombing that would spread osmium tetroxide (search), a dangerous corrosive compound. But this volatile chemical would have burned up in any explosion, scientists say.

— The long-jailed Jose Padilla (search), an American Al Qaeda member accused of planning a radioactive "dirty bomb" in the United States, is said by U.S. officials to have hoped to use uranium. But uranium has low radioactivity, and would have had no more impact than lead in a bomb, scientists note.

— Eight Algerian and Libyan defendants accused of "conspiracy to manufacture chemical weapons" were freed in London last April after authorities acknowledged tests showed a substance found in one of their apartments was not highly lethal ricin (search), as earlier alleged. The plant extract, effective as a poison dealt to individuals, was long ago dismissed by military arms-makers as an impractical mass-casualty weapon.

— American WMD specialists in Iraq reported that insurgents there last year recruited a Baghdad chemist to make the blistering agent mustard gas (search), a chemical weapon developed in World War I. They said he had the right ingredients, but he couldn't produce the compound.

The only known terrorist use of a chemical weapon occurred in 1995 in the Tokyo subway system, when Aum Shinrikyo (search) cult members punctured plastic bags of sarin (search), unleashing nerve-agent vapor that felled thousands of commuters.

The cult, including scientists, is believed to have spent millions of dollars on the demanding, dangerous production process, but came up with only impure sarin. It killed 12 people — hardly a mass-fatality terror attack, specialists point out.

"Regardless of what people say, this is very difficult to do, to inflict mass casualties with chemical or biological weapons," said Jonathan Tucker, an authority on unconventional arms with California's Monterey Institute of International Studies. "One really needs large quantities."

Oregon toxicologist Dr. Robert Hendrickson (search) calculates that terrorists would need 1,900 pounds of sarin — more than 200 gallons — to kill half the people in a typical open-air baseball stadium. So much liquid, with dispersal devices, would be extremely difficult to conceal and to produce, probably taking 10 years in a basement-sized operation, experts say.

Thousands of tons of sarin and VX nerve agent (search) already exist, in old U.S., Russian and other military arsenals. But those weapons' potency has degraded and they're being destroyed under the 1997 treaty banning them. Security around the storage sites has been tightened since the Sept. 11, 2001, U.S. terror attacks.

If true chemical weapons prove beyond their reach, experts say, terrorists may turn to far less lethal but more available pesticides and caustic compounds. Large amounts of sulfuric acid (search), the "battery acid" for sale at $2 a gallon on the Internet, were among the Jordanian group's chemicals.

"Terrorists are opportunistic," Tucker said of that group's motley collection. "They apparently figured it would produce some toxic mess that would do some harm."

The prime target in Amman was Jordan's General Intelligence Department, prosecutors said. Defense attorneys said the men admit planning a bombing, but their cache didn't include ammonium, potassium nitrate and some other compounds mentioned by prosecutors.

A televised "confession" to a chemical plot by alleged bombmaker Azmi al-Jayousi (search) was coerced, said lawyer Khreis, who contended Jordan's U.S.-aligned government was exaggerating the threat because "they want approval of people in the street and of Parliament for their antiterror actions."

Military prosecutors, who wouldn't discuss the case on the record, claim a toxic cloud killed rabbits in the desert in a test explosion of the purported chemical cache. A Jordanian army chemical expert recently testified, however, that only considerable expertise and equipment could produce a mass killer from the mix.

"A chemical bomb needs a qualified chemist," Khreis said. "Al-Jayousi has a 6th-grade education."

Some analysts say the facts of chemistry may mean little in the end for those who want to terrorize populations, as long as the word "chemical" is heard on air or seen in headlines.

"One needs only to look at the adjectives used by the media to describe chemicals to understand why the general public is frightened: toxic, killer, lethal, deadly," said Hendrickson, of Oregon Health and Science University.

Whether Internet "recipes" work or not, said the FBI's Van Duyn, "I'm not sure they need to be very effective."