It was the first time the Defense Department has announced it might have evidence of the sort of prohibited unconventional weapons program that justified forcibly disarming Saddam Hussein.
However, officials caution that the alleged mobile lab may not be a "smoking gun." The interior of the vehicle was washed down with bleach and contained no traces of biological agents, reports CBS News Correspondent David Martin.
Under Secretary of Defense Stephen 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."
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.
The trailer, painted a military color scheme, 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.
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 started March 20.
The suspect truck was being moved to Baghdad for further investigation.
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 and so on.
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.
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 the reason Saddam didn't use unconventional weapons against invading forces may be that these weapons were buried too well to retrieve before the fast coalition dash to Baghdad.
"We've collected evidence, much of it documentary, that suggests there was an active program" for unconventional weapons, Wallace said.
"A lot of the information that we're getting is coming from lower-tier Iraqis who had some knowledge of the program but not full knowledge of the program," he told Pentagon reporters in a videoconference from the Iraqi capital. "And it's just taking us a while to sort through all of that."
He did not elaborate.
Acknowledging that it was only one of his theories, Wallace said the reason such unconventional weapons never were used was that the Iraqis had to hide them from U.N. weapons inspectors up until the last days before the war.
"Inspectors only left Baghdad a few days before the start of the campaign," Wallace said. "Because they were so clever in disguising them and burying them so deep, they themselves had a problem getting to it."