They were thorough, impartial and right. So why lose their skills?

As Allied troops mobilised, the Iraqis were warned that failing to cooperate fully with the inspectors would have "serious consequences". The inspectors, for their part, found the Iraqis evasive - but uncovered no evidence that banned weapons programmes had resumed, a position they doggedly maintained as the Bush administration grew ever more impatient.

Nevertheless, the fateful decision to proceed with the invasion was taken and the troops moved in just as the inspectors pulled out. After that, UNMOVIC's activities were reduced to satellite monitoring.

Nearly five years later, the staff of the agency not only look upon that four-month inspection as their greatest test, but also as their swan song. As we all now know, they were right: Iraq had no weapons of mass destruction.

But while the inspectorate showed impartiality in the face of concerted pressure, that has not been enough to save it; indeed, some speculate that it helped seal its fate. In June, the UN Security Council announced that the monitoring agency was being wound up. While there was some discussion of finding a way to maintain the knowledge and expertise that had been gained, that came to nothing, as did talk of setting up a broad-based inspection agency that could be called upon to enter other countries as and when required.

Ewen Buchanan, from Edinburgh, was a spokesman for UNMOVIC and an adviser at the agency. He now works at the UN Office of Disarmament Affairs. A former Foreign Office official with a background in arms control, he joined the agency's predecessor organisation, UNSCOM, in 1995 and subsequently wrote the inspectors' reports for the Security Council. He says: "I think the various previous chairmen, and Blix for UNMOVIC, were able to establish a body that was impartial and operated with integrity, and that we were working for the UN and not for a country or group of countries."

However, he regrets the commission's demise. "I'm more than saddened - it's been a lost opportunity, because over the years a tremendous amount of expertise and experience has been built up. That has all now largely been lost and it would be difficult to recreate that in any way that wouldn't require a lot of time and energy."

His disappointment focuses on the so-called "roster of experts", a 350-strong group scattered across 63 countries, with a high level of expertise in various fields of technology. They were trained in weapons inspection techniques and had carefully honed "procedures, methodologies and practical experience". They could be called upon by the UN at short notice, but were not costing the organisation anything, day to day. "That roster of trained experts will wither," says Buchanan.

Brian Mullady, a former US Army colonel, was director of the information, technical support and training division of the inspectorate, and it was his responsibility to conduct a global recruitment drive, flying to places such as Nepal, Beijing, Seoul and Vienna to recruit "a tremendously capable international team". Mullady echoes Buchanan's feelings. "It bothers all of us to have these people drift away from the ability to serve international inspectorates. I really hate to see that capability gone." But, he adds, that is his personal feeling; on an official level, the question of how a successor organisation would be paid for is a difficult one, as the agency was funded by sales of Iraqi oil.

There is another consideration, as it is far from certain that other countries - Iran, Syria or North Korea, for instance - would admit entry to an agency with such an invasive mandate. Iraq let in the inspectors on pain of military action, but it is far from certain there would be the appetite for making more threats of that kind to other countries.

With the proposal for a successor organisation on ice, the inspectorate's dozen remaining staff have been winding up its operation and producing a compendium, which Buchanan characterises as "everything you want to know about Iraq's weapons programmes in 1400 pages". Its purpose has been to highlight lessons learned, and the authors have been unfailingly frank. "On the biological front, for example, we sent the world's finest weapons inspectors to a facility in Iraq," says Buchanan. "This facility was said by the Iraqis to be a single-cell protein production site, and if we'd actually taken single-cell protein experts instead, they would have seen in five minutes that the place was not configured for the stated purpose." Similarly, it became apparent that if weapons inspectors based their assessment of a facility on familiar safety features, they risked missing things. "Containment equipment that you would expect to find in another country, the Iraqis didn't care about."

That compendium has now been published, but the organisation of the agency's archive continues. And this is no ordinary archive. It contains the so-called "cookbooks" detailing such information as how to weaponise anthrax and how to make the nerve agent VX. There are also hundreds of audio and video tapes, the information dating back to UNSCOM's earliest days, and so many box files that, if laid end to end, they would stretch 1.5km.

There have been surprises, such as when, in September, staff clearing out a filing cabinet came across a vial of fluid feared to be the chemical agent phosgene, sparking a security alert around the UN headquarters in New York. Fortunately, it proved to be a harmless solvent, though there were red faces over this WMD scare at the heart of the UN.

Certainly, the agency's archive will have to be closely guarded. The Security Council has proposed that it be kept under lock and key for at least 60 years, with access only for legitimate purposes such as helping with Iraqi clean-up operations.

Peter Prosser, head of what was the biological analysis and assessment section, is closely involved with the archiving process. "It's almost like going to your own funeral," he says. "It's bittersweet, but perhaps a bit more bitter than sweet."

Some of the material relates to the work of the UNSCOM inspector Dr David Kelly, who committed suicide after being named as the possible source behind the BBC story on the "sexing up" of the government's Iraq dossier. He was a friend of Prosser's. "I tried to call David on the day he died," says Prosser. "It was a real shock to me. When you're in the field you work very closely with people, and it's almost like being married to them.

"David used to come to Iraq every month and I would ask him to come in and address my team. I would tell them that they could learn more from David Kelly in an hour and a half than I could tell them in a week and a half. He was very highly regarded."

Now the material relating to Kelly's work uncovering Iraq's bioweapons programmes in the mid-1990s must be archived along with the rest of the material, while the agency's $12m worth of assets must be disposed of. The Iraqis themselves are having first right of refusal on much of the unrestricted equipment, while other agencies will acquire items - the International Atomic Energy Agency (IAEA), for example, will receive surveillance cameras and the Organisation for the Prohibition of Chemical Weapons (OPCW) will benefit, too.

And so UNMOVIC passes into history - but some important questions will be churned up in its wake. For instance, if there were to be another weapons inspectorate in future, could a single country coordinate it, or is a multilateral organisation more desirable? Prosser says he would fear that "if there were just one country doing an inspection, it could be accused of being biased".

Mullady believes the inspectorate's record shows how effective international organisations are: "Simply look at the results in 2003, when the Americans and the British went in and found they were wrong. We'd always said there was no evidence of the WMD there at that point."

Two organisations already exist to monitor and verify the destruction of proscribed weapons: the IAEA for atomic weapons and the OPCW for chemical. But there is no such body for missiles, and although there is a Biological Weapons Convention, it has no verification body because discussions on setting one up foundered at the UN. Professor William Walker, an arms control expert at the school of international relations at St Andrews University, is clear on where the blame lies for this: "The Americans pulled the rug out from under it - they dislike multilateralism and these kind of treaties.

"There is an intrinsic problem with monitoring bioweapons, because they are so easy to make," adds Walker. "But at least with a verification body we'd have something to work with."

These so-called WMD, he says, "certainly scare people a lot, and there's a potential for mass casualties". But the threat they pose has also been hyped. "To some extent, the claims about the terrible destructive effect of these weapons was used to legitimise a war. When we look back on it, we'll see that."

Under pressure: a history of the inspectors

• UNMOVIC, the UN Monitoring, Verification and Inspection Commission, was established by the UN Security Council in December 1999 in the hope that it would attract greater cooperation from Iraq than its predecessor, the UN Special Commission on Iraq (UNSCOM).

• UNSCOM had been set up following the Gulf War ceasefire in 1991, when the Security Council demanded that Iraq give up its nuclear, chemical and biological weapons capabilities, and its weapon delivery systems with a range of over 150km. UNSCOM, along with the International Atomic Energy Agency (IAEA), was charged with ensuring that it complied with these requirements.

• This proved successful: it uncovered an offensive biological weapons programme, VX nerve agent capability, a clandestine nuclear programme and long-range missiles capable of delivering WMD.

• But the inspectors faced persistent Iraqi non-cooperation, making it impossible to verify Iraqi disarmament and put in place an ongoing monitoring and verification system to ensure banned weapons programmes did not resume. The inspectors withdrew in December 1998, in advance of air strikes designed to compel the Iraqis to cooperate.

• UNMOVIC was then set up, acquiring all UNSCOM's assets and archive, plus some staff, and in November 2002 it finally went into Iraq. By mid-December, the inspectors were carrying out eight inspections per day.

• Dr Blix told the Security Council on March 7, 2003, that there were unanswered questions, particularly relating to unaccounted-for anthrax and VX. However, no chemical or biological weapons were uncovered. Iraqi documents submitted before the inspections revealed only two surface-to-surface missiles capable of exceeding 150km.

• The US and UK put pressure on UNMOVIC, with US Secretary of State Colin Powell telling the Security Council Iraq was hiding weapons and deceiving inspectors. Following the invasion, nothing was found.