U.S. forces in Iraq continued Thursday to examine what they believe was a mobile weapons lab, hoping to find concrete evidence that proves Saddam Hussein possessed weapons of mass destruction.
The Pentagon confirmed Wednesday that a trailer found in the Iraqi desert matches the description of a mobile biological weapons (search) laboratory.
American forces are now conducting tests to determine whether the trailer may be the "smoking gun" that proves Iraq had an ongoing program to develop weapons of mass destruction.
"On the smoking gun, I don't know," Under Secretary of Defense Stephen Cambone (search) said, when asked whether this was a breakthrough in the continuing coalition search for Saddam Hussein's biological and chemical weapons.
Cambone said that what the U.S. military has in its possession is the kind of mobile laboratory that Secretary of State Colin Powell described in a report to the U.N. Security Council as he sought to justify forcibly disarming Saddam.
"They have not found another plausible use for it," Cambone said.
The information Powell gave the U.N., Cambone said, "was based on information from a number of sources and it confirms what the source said."
Fox News reported Tuesday that a vehicle found by Kurdish fighters last week in the northern Iraq city of Irbil (search) may be a mobile weapons laboratory. Senior defense officials told Fox News then that they were "confident" the vehicle was used to manufacture biological or chemical weapons agents.
The officials said the possible mobile biological weapons lab was moved to the Baghdad International Airport for further testing.
Cambone said that experts had done initial tests on a trailer taken into custody April 19 at a Kurdish checkpoint in northern Iraq but said that more substantial testing is required.
Cambone noted that the vehicle's surface had been washed with a caustic material and it likely would have to be dismantled before testing could be done on hard-to-reach surfaces.
The truck and its contents were still being swabbed for any trace at all of bioagents.
"There are a number of tests going on right now in a number of different locations in regards to Iraqi weapons of mass destruction," officials told Fox News.
Painted in a military color scheme, it was found on a transporter normally used for tanks and -- as an Iraqi defector has described Iraq's mobile labs -- contains a fermentor and a system to capture exhaust gases, Cambone said.
"While some of the equipment on the trailer could have been used for purposes other than biological weapons agent production, U.S. and U.K. technical experts have concluded that the unit does not appear to perform any function beyond what the defector said it was for, which is the production of biological agents," Cambone said.
Earlier Wednesday, Lt. Gen. William Wallace said that American forces have collected "plenty of documentary evidence" suggesting that Saddam had an active program for weapons of mass destruction.
Wallace, commander of the Army's V Corps, told a press conference that Saddam may not have used them in Operation Iraqi Freedom because they were buried too well to retrieve before the coalition made it to Baghdad.
The Bush administration said destroying Iraq's suspected chemical, biological and nuclear weapons programs was the main reason for the war. Despite weeks of searches of suspected sites, nothing conclusive has been reported found so far.
And although Pentagon officials suggested before the war that some Iraqi units were armed with chemical weapons, none was found when those units were overrun.
If proven to be a mobile lab, the trailer would be the first discovered in the military campaign that began March 19.
On several occasions, troops have found substances they said tested positive as nerve agents or other chemical weapons materials, only to learn from more sophisticated testing that they were crop pesticides, explosives or similar substances.
Coalition troops have also come upon at least one other vehicle they thought was a mobile weapons lab, yet tests proved those guesses as inconclusive.
A defense official said before Cambone's press conference that he and others "feel good" about the prospect this time that they have found good evidence of an unconventional weapons program.
But they are being careful to cover all bases. He noted that many questions will be asked if it is announced as evidence -- including "chain of custody" information on who has handled the truck and whether it might have been tampered with.
The vehicle resembles an 18-wheeler Powell said in a Feb. 5 presentation before the U.N. Security Council was a mobile weapons lab that had been moved around to elude weapons inspectors. That was Washington's final large push to persuade the Security Council of the need to forcibly disarm Iraq.
Powell presented recently declassified intelligence, including satellite photos and radio intercepts, to support the U.S. case that Iraq had defied all demands that it disarm and had links to Al Qaeda and other terrorist groups.
Fox News' Bret Baier, Ian McCaleb and The Associated Press contributed to this report.