American forces are changing their search strategy after coming up empty at most of the top suspected weapons sites in Iraq, officials said Wednesday.

And the White House appeared to be trying to scale back expectations that weapons of mass destruction will be found.

Troops on the ground have searched more than 80 sites that prewar U.S. intelligence judged the most likely hiding places for chemical and biological weapons as well as evidence of an Iraqi nuclear program, Defense Department officials said on condition of anonymity.

There are more than 1,000 suspected sites but 100 or so were the searchers' top priority.

Some analysis is pending on some substances found. But finding no stockpiles of chemical or biological agents after more than a month into the campaign, teams are now setting aside the search list and deciding where to go more on the basis of new information from Iraqis, three defense officials said Wednesday.

"We did have several hundred sites that we had some history of intelligence on that we were going to exploit," said Lt. Gen. David McKiernan, commander of land forces in Iraq. "This regime over the last decade has been pretty good at hiding material and moving it around, so it was no surprise to any of us that many of these sites that we've already exploited have not necessarily turned up the material."

Two other officials said that in recent days officials have realized the list is of questionable value because of the ability of the Iraqis to destroy or remove weapons and equipment.

Although U.S. officials say the war is not over, more forces are headed home. The aircraft carrier USS Harry S. Truman and its battle group of 11 ships has been released from wartime duty and is scheduled to arrive at its home port of Norfolk, Va., in May, officials said Wednesday. The Truman left Norfolk Dec. 5 and originally was to complete its sea duty in early June.

The Truman is now in the Mediterranean Sea, along with the USS Theodore Roosevelt carrier battle group. Two other carriers, the USS Kitty Hawk and the USS Constellation, left the Persian Gulf last week. The only carrier now in the Gulf is the USS Nimitz.

Also, two of the three Marine Expeditionary Units that fought in Iraq are preparing to head home. The 24th Marine Expeditionary Unit will soon be headed back to Camp Lejeune, N.C., and the 15th Marine Expeditionary Unit will follow a short time later en route home to Camp Pendleton, Calif., officials said. Both units have about 2,200 people.

The sites in Iraq searched for chemical and biological weapons have included mosques, homes, factories and government ministries. In some cases teams arrived to find buildings completely empty -- swept of any evidence, one official said.

One search team also interviewed an Iraqi scientist last week who said some weapons were moved to Syria and others were destroyed before the war. His account has not yet been verified.

McKiernan called the search "ad hoc" now, meaning troops will move on information culled from various intelligence, which could include captured Iraqis, documents and other sources.

Another official said the Pentagon still intends eventually to search all of the more than 1,000 possible sites, which he called "guesses" based on satellite data, other surveillance, information gathered by United Nations weapons inspectors over the years, from Iraqi defectors and elsewhere.

The prewar list did not reflect an intelligence failure, he said. But rather, moving to the new system is a natural evolution of the hunt, now that American officials are inside Iraq and can speak to Iraqis who have knowledge of weapons programs.

The existence of weapons of mass destruction -- and goal of disarming Iraq -- were the mains reasons given by the administration for the war, which did not get U.N. approval.

Hans Blix, the U.N.'s chief weapons inspector, commented Tuesday on the lack of U.S. findings.

"It is conspicuous that so far they have not stumbled upon anything," he said in New York.

White House spokesman Ari Fleischer said Tuesday there was "no question we remain confident that WMD [weapons of mass destruction] will be found."

On Wednesday he said the president still believes weapons exist there. Asked what will happen if none are found, he said "the chances of success depend not on finding something by bumping into it," but on information provided by Iraqis involved in the programs.

Asked if he meant searches might not find the weapons but rather some kind of evidence they previously existed, Fleischer said: "There are no changes in the American position. We have high confidence that Iraq did indeed have weapons of mass destruction ... that indeed will be found in whatever form it is."