Al-Qaeda-linked terrorists planned a chemical attack on Jordan's spy headquarters that could have killed 20,000 people, officials have said.
Earlier this week King Abdullah said a massive attack had been thwarted by a series of arrests, but named no target.
Now unnamed officials say the suspects have confessed to plotting to detonate a chemical bomb on the Amman HQ of the Intelligence Services.
The plot was reportedly hatched by al-Qaeda suspect Abu-Musab al-Zarqawi.
Washington has accused the 38-year-old Jordanian radical of masterminding a string of spectacular suicide bombings in Iraq.
An official involved in the inquiry in Jordan told AFP news agency: " We found primary materials to make a chemical bomb which, if it had exploded, would have made nearly 20,000 deaths ? in an area of one square kilometre.
"The target of this bomb was the headquarters of the Intelligence Services ," situated on a hill in the western suburb of Amman, he added.
The official said another operation planned by the network was to use " deadly gas against the US embassy and the prime minister's office in Amman ? and other public buildings in Jordan".

Jordan's King Abdullah revealed on Saturday that vehicles reportedly containing chemical weapons and poison gas that were part of a deadly al-Qaida bomb plot came from Syria, the country named by U.S. weapons inspector David Kay last year as a likely repository for Iraq's weapons of mass destruction.
" It was a major, major operation. It would have decapitated the government ," King Abdullah told the San Francisco Chronicle.
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King Abdullah said that trucks containing 17.5 tons of explosives had come from Syria, though he took pains not to implicate Syrian President Bashir Assad in the al-Qaida plot, saying, " I'm completely confident that Bashir did not know about it ."
In his testimony before Congress last year, weapons inspector Kay said U.S. satellite surveillance showed substantial vehicular traffic going from Iraq to Syria just prior to the U.S. attack on March 19, 2003.
While Kay said investigators couldn't be sure the cargo contained weapons of mass destruction, one of his top advisers described the evidence as "unquestionable."
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A car belonging to the al-Qaida plotters, containing a chemical bomb and poisonous gas, was intercepted just 75 miles from the Syrian border.

Supporters of outlawed Shiite cleric Moqtada al-Sadr say they expect US forces to attack the holy city of Najaf after mediation efforts failed today, while coalition officials were hopeful talks in Fallujah would end a bloody stand-off between US troops and Sunni Muslim rebels.
Sadr and his supporters said mediation efforts with the US -led coalition had failed in Najaf and they were preparing for what they believed to be an imminent US attack.
Talks halted " because the mediators have told us the Americans are putting obstacles towards finding solutions to the crisis and the situation is getting worse ," Qais al-Khazaali, the head of Sadr's office, said.
" We are expecting the Americans to attack Najaf any moment now ."

Gunmen opened fire on a US convoy near the Spanish base at Diwaniyah, east of Najaf, today and US forces fired back, a spokesman for the Spanish contingent in the occupation force, said.
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US Marines repositioned some of their forces to allow cars to reach the general hospital in Fallujah as part of confidence-building measures, an Iraqi mediator said.

A US soldier was killed and two wounded when their patrol hit an anti-tank mine near the Iraqi city of Tikrit, the military said today.
" One 1st Infantry Division soldier was killed and two wounded when their patrol struck an anti-tank mine near Tikrit at around 10.45am April 16 ," the military said.
" The wounded were evacuated to a nearby military medical facility and were listed in stable condition ."

The US led coalition in Iraq has announced the closure of all major highways into and out of Baghdad.
Coalition spokesmen have denied the closure has anything to do with threats by insurgents.
The statement said highways one and eight to the north and south of Baghdad had become targets for anti-coalition forces and were damaged and too dangerous to travel. They would be closed indefinitely.
The main highway to the west, which runs through Fallujah, has been closed for a week now.
Military spokesman Mark Kimmitt says motorists would be directed to alternate roads.
He dismissed suggestions that the closure was due to posters and flyers being distributed around Baghdad warning people to stay at home this week because of a major offensive to be launched by insurgents and said if the fight were taken to the capital, the First Cavalry Division would be waiting.