U.S. weapons hunters are re-examining the only discovery the Bush administration has cited as evidence of an illicit Iraqi weapons program (search— a pair of trailers the CIA said were laboratories for making biological weapons, senior military officers involved in the hunt told The Associated Press.

 The two metal flatbeds stocked with cooling equipment, a water tank, an air compressor and a battered fermenter were first described by Iraqi defectors as part of a weapons program. But that assertion, challenged by some U.S. defense analysts, has become the latest prewar intelligence called into question.

In six months of searches, no biological, chemical or nuclear weapons have been found to bolster the administration's central case for going to war: to disarm Saddam Hussein  of suspected weapons of mass destruction.

Although Bush administration officials continue to say publicly that the trailers were part of a biological weapons program, David Kay (search), the CIA representative charged with leading the weapons search in Iraq, acknowledged Thursday that those findings are "still very much being examined."

"We have not yet been able to corroborate the existence of a mobile BW (biological weapons) production effort," Kay told a Congressional hearing.

In fact, a re-examination of the trailers has been under way for several weeks in Iraq, led by a CIA representative, the senior military officers told AP, speaking on condition of anonymity.

Photographs of the trailers on the CIA Web site reveal few details but a more recent photo obtained by AP this week offers physical proof of the re-examination. The trailers, which were found in April and May, remain at Baghdad's airport, where the weapons teams are based and where the review is being conducted.

In a paper issued May 28, the CIA called the trailers "the strongest evidence to date that Iraq was hiding a biological warfare program." But intelligence analysts from the State Department and the Defense Intelligence Agency have said they believe the trailers were probably used to fill hydrogen weather balloons.

Kay said an investigation of the trailers has "yielded a number of explanations including hydrogen, missile propellant and BW production but technical limitations would prevent any of these processes from being ideally suited to these trailers."

Military scientists who analyzed the pair of trailers during the summer doubted they were designed to function as mobile laboratories, according to the three military officers involved in the weapons hunt.

"There were some people who really believed they were for making hydrogen for weather balloons. Almost no one was certain they were biological weapons," said one senior-ranking military commander involved in the search. "The trailers are great examples of dual-use but that's about it," the commander said.

Dual-use items, which could have either military or civilian applications, long troubled U.N. weapons inspectors who tagged most such equipment and kept it under monitoring while they were in Iraq. Defense officials in Baghdad and Washington have said much of what weapons inspectors have found so far is equipment and facilities with dual-use capabilities. There is no indication that any of those materials or places are new or were unknown to U.N. inspectors.

One of the U.S. scientists involved in the hunt, who spoke on condition of anonymity, said some conducting the search believe the Iraqis could have tried to produce biological warfare agents inside the trailers but not very well.

Also, it would have been hard, if not impossible, to hide the evidence. No traces of anthrax or any other warfare agent have been found during more than a half-dozen tests on the trailers.

Last month, Vice President Dick Cheney (searchrepeated the claim that the two trailers were "mobile biological facilities" that could have been used to make several biological agents, including smallpox.

One of the central arguments used by the CIA to support its initial findings is that one trailer had a fermenter. Smallpox, however, isn't grown with a fermenter and experts say it would be impossible to produce this specific virus in a trailer.

"There's no way that these particular labs could have been used to make smallpox," said Jonathan Tucker, a weapons expert at the Monterey Institute of International Studies who authored "Scourge," a recent book on smallpox.

 

"Smallpox can only replicate inside cells, so you need a bioreactor, not a fermenter, which is a much more sophisticated piece of equipment."

In addition, he said, smallpox would need to be grown in a maximum containment laboratory, "not in a trailer with canvas siding. If there had been a leak, it would have spread smallpox all over the country."

Some outside weapons experts who have examined photographs and the CIA report have also left open the possibility that the trailers could be for designed for conventional uses such as decontamination or fuel regeneration.

With the U.S. search effort now taking twice as long to look as U.N. inspectors had this past winter, some within Congress are becoming increasingly skeptical that any weapons of mass destruction will be found.

Once confident, Senate Intelligence Committee Chairman Pat Roberts, R-Kan., said Wednesday he was not so sure any more.

"I think it's such a tough job," he said. Asked if he believed the weapons ever existed, Roberts said, "At one point I'm sure they did. Where they are now and what point they are now, I just don't know."

Roberts' committee and its House counterpart are investigating the prewar intelligence. In a letter last week to CIA Director George Tenet, the two top members of the House panel said there were "significant deficiencies" in the collection of intelligence on suspected Iraqi weapons programs and any ties Iraq may have had with al-Qaida terrorists.

In his Feb. 5 presentation to the U.N. Security Council, Secretary of State Colin Powell used some of that intelligence to lay out the U.S. case for war. He displayed artists' conceptions of mobile weapons facilities.

Powell stood by his assertions during an appearance Sunday on ABC.

"Even though there are differences within the overall intelligence community, the director of central intelligence examining all of the material with respect to that van and examining counterarguments as to what it might be stands behind the judgment that what we found was positive evidence of a mobile biological weapons lab and (it has) not been discounted sufficiently."

The original tip on the trailers was provided by a defector working with Ahmad Chalabi, the head of the Iraqi National Congress and now a member of the U.S.-appointed Governing Council in Iraq.