One scenario in Iraq goes like this: Insurgents finally succeed in concocting chemical weapons and use them against U.S. troops. Not only could it happen, it nearly did, American arms investigators say.

They say Iraqi resistance groups have tried to manufacture "CW," and one might have managed it if the Americans hadn't swooped down on them.

The danger has even spilled over into Jordan, where authorities say a plot hatched in Iraq aimed to kill thousands with "poison clouds." The threat demands "sustained attention," says the chief U.S. arms investigator.

The insurgents' work on chemical arms was disclosed in the final report of Charles A. Duelfer's Iraq Survey Group (search), the account of its fruitless 18-month hunt for weapons of mass destruction in Iraq.

In a little-noted annex of the 350,000-word document, the joint CIA-Pentagon teams tell of having broken up an insurgent group last June that for six months tried to make weapons agents.

The group had recruited a Baghdad chemist and obtained chemicals from farmers who looted state companies and from shops in Baghdad's chemicals market, the report said.

They first tried to make tabun (search), a nerve agent, but couldn't get the ingredients. Then the chemist, who had no weapons-making experience, was unable to manufacture the blistering agent mustard (search), although he had the right chemicals, the report said.

The insurgents hired another chemist, who succeeded in making ricin (search) base, a poisonous plant extract, from castor beans, but at that point a U.S. raid on the laboratory, at Baghdad's al-Abud trading complex, disrupted the network.

The raiders did not capture leaders and financiers of the "al-Abud network," who the report said included Sattam Hamid Farhan al-Gaaod (search), an international trader said to have been close to ousted president Saddam Hussein. The insurgents also apparently took away nine mortar rounds that had been loaded with the insecticide malathion (search), the report said.

The U.S. command in Baghdad says no further progress has been made tracking the group since the Duelfer report was issued last October.

"The most alarming aspect of the al-Abud network is how quickly and effectively the group was able to mobilize key resources and tap relevant expertise to develop a program for weaponizing CW agents," the report said.

It said that with time the insurgents might have mastered weapons-making, with "devastating" consequences for U.S. forces.

The Duelfer account also said various sources reported insurgents were trying to produce chemical weapons elsewhere in Iraq.

In November, U.S. and Iraqi forces retaking the city of Fallujah reported apparent evidence of that: an insurgent "chemical lab" where they found the poisonous industrial compound hydrogen cyanide and a book of instructions for making potential chemical weapons.

Weeks earlier, in mid-October, Jordanian authorities said they had foiled planned chemical attacks by suicide bombers on Jordanian government targets. They said the plot was conceived by alleged terrorist leader Abu Musab al-Zarqawi (search), a Jordanian fugitive in Iraq. Nine alleged accomplices are on trial in Amman.

The prosecution claims the explosions would have created "poison clouds" and killed thousands. The compounds reported seized were mostly ingredients for explosives, however, not chemical weapons, although the list includes "pesticides," possibly a weak substitute for more effective chemical agents.

The Duelfer report warned that chemical weapons-makers from the old Iraqi regime might help insurgents make more sophisticated agents. The U.S. government has been planning to keep Iraqi weapons scientists occupied with nonmilitary projects, but that program thus far involves only 125 of them, of an estimated 500 targeted.

Some outside experts suggest the likelihood and impact of insurgent-brewed weapons may be overstated.

Not only are chemical weapons difficult to make, but "they are notoriously difficult to use," said John Eldridge, editor of Jane's Nuclear, Biological and Chemical Defence.

"They use rocket launchers," this British expert said of the insurgents, "and trying to put a chemical warhead into a rocket is pretty difficult."

British researcher Richard Guthrie, of Sweden's Stockholm International Peace Research Institute (search), also cited the difficulty of working with the deadly nerve agent sarin (search), which is colorless and odorless, requiring advanced equipment to know when it is leaking.

Neither saw much tactical need for the insurgents to go to such lengths, but Guthrie saw a possible psychological edge.

"A lot of high-level attention is paid to chemical and biological terrorist issues," he said. "If you're an insurgent group planning priorities and tactics, and the signal is sent that this is what people are scared of, you put more effort into doing that."