The trailer was similar to one seized last month that U.S. officials believe may have been a germ weapons workshop for the Iraqis, two officials said, speaking on condition of anonymity.
American troops found the second trailer around the northern city of Mosul Friday night or Saturday morning, one official said. U.S. experts are examining it in Mosul before sending it to the Baghdad airport, where the first trailer is being held, the officials said.
Weapons experts aren't sure if the latest trailer is connected with the first, but it appears to have many of the same components, one official said.
The first trailer was painted in a military color scheme and mounted on a transporter usually used for tanks, the Pentagon's intelligence chief said last week.
The trailer contained a device for fermentation, or growing microbes, and a system to capture exhaust gases from that process, said Stephen Cambone, the undersecretary of defense for intelligence.
It was unclear what purpose the first trailer would have served other than a mobile lab, but intelligence officials told The Washington Post that it was unclear if the lab had ever actually been used to create toxins and, if so, when.
The vehicle had been cleaned with a caustic substance, however, so no toxic agents were found.
The second trailer could have been for missile maintenance or agricultural work, according to experts quoted by the New York Times.
If proven to be weapons labs, the trailers would constitute the first finds of the illegal weapons that were the rationale for the U.S.-led war.
Secretary of State Colin Powell said in Egypt Monday he expected U.S. teams to find weapons of mass destruction in Iraq.
In testimony to the United Nations in February, Powell said four human sources had led the U.S. to believe Iraq had seven mobile labs, comprising around 18 trucks, capable of producing anthrax and botulinum toxin.
"We believe there are weapons of mass destruction and we presented last week some evidence of one such program," Powell said, referring to the first trailer.
But the failure to find the vast stockpiles of chemical and biological weapons so far has led U.S. officials to suggest that Saddam Hussein ordered the weapons destroyed or hidden before the war.
The main military search team — the 75th Exploitation Task Force — is preparing to hand off the search to newly created Site Survey teams comprising some 2,000 people.
Those teams are charged with investigating everything from potential war crimes committed by Saddam's regime to alleged terrorist connections with al Qaeda.
But the change in command will not mean beefing up the size of the search teams. In fact, two officials said the field teams — under pressure from Washington and overburdened by a mammoth list of suspected weapons sites — could become smaller.
Most of the sites deemed "high priority" by the unit have been searched without success, but hundreds more sites are on the list.
Some team members and even officials back in Washington have begun to doubt actual weapons will be found. Instead they now talk of finding evidence of infrastructure that could have been used to quickly produce unconventional weapons.
Poor intelligence and a lack of cooperation by Iraqi scientists are blamed for the failures so far.
But the hunt for weapons might be helped by the surrender during the weekend of Dr. Rihab Rashid Taha, a top official in Iraq's biological weapons program called "Dr. Germ" by U.N. inspectors.
If she cooperates with her captors, Taha could provide details about Iraq's germ weapons program. U.N. inspectors have described her as unpleasant and uncooperative, but she told the British Broadcasting Corp. this year that Iraq had ended its biological weapons program.
Meanwhile, U.S. experts are poring through millions of pages of documents found in Iraq, looking for information about the country's suspected biological, chemical and nuclear programs. Investigators have found warehouses full of such documents, the senior Pentagon official said.