WASHINGTON, 6 April 2003 — US forces in Iraq on Friday found sites and substances they described as suspected components of a forbidden Iraqi weapons program. But the discoveries that US troops displayed, and the manner in which they were described at a Central Command briefing, struck experts in and out of government as ambiguous.

Iraq has the most extensive petrochemical industry in the Middle East and a wealth of vaccine factories, single-cell protein research labs, medical and veterinary manufacturing centers and water treatment plants. Nearly all of them are dual-use facilities, capable of civilian or military employment, but most were devoted to legitimate activity even at the height of Iraq’s secret weapons programs.

Moving warily through that industrial landscape, US and allied ground forces will inevitably find, as UN inspectors have found since 1991, thousands of potential weapons sites but few, if any, that could be nothing else. Iraq’s continued concealment of such weapons is the allegation at the core of the Bush administration’s case for war. If the hunt for them relies on that sort of survey, experienced investigators said, it faces a long road to an uncertain result.

In the first of Friday’s discoveries, the 3rd Infantry Division entered the vast Qa Qaa chemical and explosives production plant and came across thousands of vials of white powder, packed three to a box. The engineers also found stocks of atropine and pralidoxime, also known as 2-PAM chloride, which can be used to treat exposure to nerve agents but is also used to treat poisoning by organic phosphorus pesticides.

“It is clearly a suspicious site,’’ Col. John Peabody, commander of the division’s engineering brigade, told an Associated Press reporter embedded with his unit.

Peabody was right about the site being suspicious. UN inspectors have surveyed Qa Qaa some two dozen times, most recently last month. But some 1,000 structures there, organized into 10 or more factory complexes, have mainly been devoted to such conventional military industries as explosives and missile fuels. Neither is forbidden under UN Security Council mandates. Qa Qaa was last linked to proscribed activity in 1995 — and somewhat peripherally then.

“Based on (the powder and antidotes) you couldn’t form any real judgment,’’ said Terence Taylor of Britain, a former inspector with the UN Special Commission (UNSCOM). “It is a place where there would be a lot of chemicals, not necessarily related to chemical or biological weapons.’’ Brig. Gen. Vincent Brooks, briefing reporters at Central Command headquarters in Qatar, said he had no details on the suspicious powder but said that “certainly it’s an item of interest.’’

But Brooks volunteered another discovery in western Iraq. Special Operations troops raided a building there “that we think now was probably an NBC training school,’’ he said, referring to nuclear, biological and chemical weapons.

Brooks said military commanders based that belief on a shelf of clear- and brown-glass bottles with yellow labels. “Can we bring up the bottles?’’ he directed, calling for a photograph in his slide show.

“This is what we saw,’’ he said. “One of them had been marked ‘Tabun,’’’ a nerve agent developed during World War II. The bottle, he said, contained a few drops of liquid. He did not say whether tests showed the liquid to be the deadly chemical. But a quantity measured in milliliters, US government and UN experts said, probably would not constitute a material breach of that obligation.

“There will be more and more alarms like this,’’ said an expert on nerve poisons with long experience in Iraq. “If you have a vial marked ‘Tabun,’ it could be simulant, it could be a sample used for training purpose or for evaluation of protection equipment....The regular procedure is to put one drop on the surface of a material used in the production of a protection suit, and you analyze the penetration of this drop through the material.’’

Chemical protection gear is permitted under the UN restrictions. A serious find, the expert said, would involve “Tabun in a munition, or in bulk storage, or traces of Tabun in a reaction vessel’’ of the sort used to manufacture the agent in quantity. At part of the Qa Qaa facility, where the white powder appeared on Friday, UNSCOM ordered the destruction of reactors, heat exchangers and storage vessels suspected of chemical weapons production.

The International Atomic Energy Agency also reported, after a 10-day visit in September 1995, that Iraqi scientists acknowledged involvement of the Qa Qaa facility “in support of the development of the implosion package’’ intended to detonate a nuclear weapon if Iraq acquired highly enriched uranium.

US officials overseeing the weapons hunt generally do not expect to make major finds at sites previously identified as suspicious, noting that Iraq’s documented history of concealment relied on constant movement. “There’s lots we don’t know about current whereabouts,’’ Douglas Feith, undersecretary of defense for policy, said in an interview. “That’s going to be true until we have full control of the country, and even a time thereafter.’’

He added: “If you have (custody of) the people who know everything, and they tell you everything they know, then you could learn the whereabouts of everything very quickly. But if those people aren’t around, or they’re dead, or they’ve organized things in such a way that nobody has too much knowledge, it’s going to take you awhile.’’