AS THEY advanced cautiously through the streets of al-Aziziyah, a small Iraqi town on the northern banks of the Tigris River, a detachment from the 1st Division of the US Marines was approached by a local man who seemed anxious to direct them to a nearby school for girls.

The marines suspected a trap but the man, who spoke no English, grabbed a gas mask hanging from a soldier’s belt and pointed again at the school. Eventually, an interpreter learnt that two months earlier Iraqi soldiers had visited the school at night, buried something in a courtyard and covered it with an expanse of concrete.

Within hours the marines were digging for what some assumed was a cache of chemical weapons. General James Mattis, their divisional commander, turned up to inspect the work. A day later the concrete cracked and the marines recovered their prize: a pair of small missiles. The warheads bore chemical markings but contained only conventional explosives.

It was one of many let-downs in the American and British drive to produce evidence of illicit Iraqi weapons of mass destruction (WMD). Five weeks after the first bombs were dropped, not a single conclusive trace has been found of the 100-500 tons of toxic agents that Colin Powell, the US secretary of state, described in February as a “conservative estimate” of Iraq’s chemical stockpile.

The search for proof of Iraqi deception has prompted an acrid political debate about the quality and reliability of the intelligence amassed by London and Washington. Unless a captured Iraqi official suddenly delivers the key to a bunker stacked with anthrax, questions are likely to linger about the long list of allegations that were presented to the United Nations as justification for war.

“It may be that there weren’t as many weapons as (President George W Bush) said, in which case we have a major intelligence failure and a huge blow to US credibility,” said Joseph Cirincione, an arms expert at the Carnegie Endowment for International Peace in Washington. “The other option is that there really are as many weapons as the president feared and they are no longer under anyone’s control.”

In his first interview since the fall of Baghdad, Bush disclosed on Thursday night that coalition forces had so far searched only 90 out of “literally hundreds” of suspected weapons sites. “It’s going to take time to find them,” he said. “Whether (Saddam) destroyed them, moved them or hid them, we’re going to find out the truth.”

Other officials were not so sure. Many of the sites visited had been at the top of the CIA’s list of most promising targets.

Bush said a few sites had provided materials and substances that were still being examined. However, there has been no sign of the mobile chemical laboratories, drones fitted with poison sprays or Scud missile launchers in the western desert — not to mention 5,500 gallons of anthrax, several tons of VX gas nerve agent and numerous other toxins — that Powell had likened to a “submerged iceberg” of illicit Iraqi weaponry.

Instead, the coalition forces have suffered a long sequence of false alarms, wild rumours and inconclusive maybes. A supposed chemical weapons plant near Najaf yielded nothing. Vials of white powder discovered at Latifiyah contained only explosives. Barrels of suspected sarin and tabun, both lethal nerve agents, turned out to be pesticide.

When a dozen soldiers from the US 101st Airborne Division were struck by vomiting and dizziness after visiting a suspected chemical weapons site, it turned out they had inhaled only noxious fertiliser fumes.

For Bush, who continues to bask in seemingly impregnable approval ratings, the failure to find WMD threatens only passing domestic embarrassment. But he may find it harder to convince potential future allies of the reliability of American intelligence on North Korea or Iran. And a rougher ride beckons for Tony Blair, with some members of the Labour party determined not to let the WMD issue drop merely because the war has ended well.

“For the Americans the war was all about liberation first and then finding WMD,” said one official in London. “For us it was finding WMD and then liberation.”

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