"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 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 produce, probably taking 10 years in a basement-size operation, experts say.
Thousands of tons of sarin and VX nerve agent 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. terrorist attacks.
If true chemical weapons prove beyond reach, experts say, terrorists may turn to far less lethal but more available pesticides and caustic compounds. Large amounts of sulfuric acid, 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 key target in Amman was Jordan's General Intelligence Department, prosecutors said. Defense attorneys said the men admitted 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 bomb maker Azmi al Jayousi was coerced, said lawyer Khreis, who contended that Jordan's U.S.-aligned government was exaggerating the threat because "they want approval of people in the street and of Parliament for their anti-terror actions."
Military prosecutors, who wouldn't discuss the case on the record, contend that a toxic cloud killed rabbits during a test explosion of the purported chemical cache in the desert. 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 sixth-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 broadcast or used 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 the 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."