Washington ? The discovery of two Iraqi truck trailers equipped with fermenters is the strongest evidence yet that Saddam Hussein had a biological weapons program, a U.S. intelligence report said Wednesday. But officials still have found no such weapons.

The report by the CIA and the Defense Intelligence Agency comes as the Bush administration faces pressure to prove its justification for the war — that Saddam had to be disarmed of nuclear, biological and chemical weapons programs that were an imminent danger.

“Examination of the trailers reveals … an ingeniously simple, self-contained bioprocessing system,” the report says. “Both trailers we have found probably are designed to produce BW agent in unconcentrated liquid slurry.”

“BW” is for “biological weapons.” U.S. officials believe Iraqi leaders were interested in developing mobile fermentation units for such weapons, but the report does not verify Iraq had the weapons, possibly ready for battlefield use.

The report’s conclusions key on evidence from an Iraqi source, a chemical engineer who claims to have managed one of the mobile production plants.

That source has also identified photographs of the captured trailers, the new report says. Three other Iraqi sources also described efforts by Saddam’s government to build a mobile biological weapons plant.

Intelligence officials who briefed reporters Wednesday on condition of anonymity, said they had considered and discarded possible legitimate uses for the trailers, ranging from the production of hydrogen to pesticides. The trucks would not be able to do any of those very well, they indicated.

“The Iraqis had a motivation to inefficiently produce a biological agent,” said one intelligence official. “They had no motivation to inefficiently produce anything else.”

Captured Iraqi scientists have said the vehicles were for producing hydrogen for weather balloons in support of artillery. The report acknowledges the trailers could be used to make hydrogen but says the effort would be inefficient compared with widely available commercial hydrogen generation systems.

A technical analysis of the trucks showed they could be used, though inefficiently, to complete the early steps of creating a biological weapon, intelligence officials said. Based on information from the source, they believe each truck was part of a two-truck or three-truck system that could create a finished agent.

Each of those multiple-truck systems — the CIA believes Iraq had around seven — could produce a few pounds of biological agent a month. The intelligence officials said they do not know which agent — anthrax, botulinum toxin or others — might have been involved.

Information about the trailers, based largely on the Iraqi engineer’s description, was a key component of Secretary of State Colin Powell’s February 2003 presentation to the United Nations regarding Iraq’s alleged weapons programs.

There is no evidence the two trailers were ever actually used to make biological weapons, the intelligence officials said.