WASHINGTON — Contradicting one of several arguments for a war that has cost more than 1,000 American lives, the top U.S. arms inspector said Wednesday he found no evidence that Iraq produced weapons of mass destruction after 1991. He also concluded that Saddam Hussein’s ability to develop such weapons had dimmed during a dozen years of sanctions before last year’s U.S.-led invasion.

But Hussein still wanted to pursue weapons of mass destruction and hoped to revive his weapons program, interviews with the imprisoned dictator revealed.

Contrary to prewar statements by President Bush, Hussein did not have chemical and biological stockpiles when the war began, and his nuclear capabilities were deteriorating, said Charles Duelfer, head of the Iraq Survey Group.

The findings come less than four weeks before an election in which Iraq is a central issue. They could boost Democratic candidate John Kerry’s contention that Bush rushed to war based on faulty intelligence and that United Nations sanctions and weapons inspectors should have been given more time. Both Kerry and his running mate, however, voted to use force against Hussein.

Duelfer supports Bush’s argument that Hussein remained a threat. Interviews with the toppled dictator and other former Iraqi officials made clear that Hussein still wanted to pursue weapons of mass destruction and hoped to revive his weapons program if U.N. sanctions were lifted.

“What is clear is that Saddam retained his notions of use of force and had experiences that demonstrated the utility of WMD,” Duelfer told Congress.

Campaigning in Pennsylvania, Bush defended the decision to invade.

“There was a risk, a real risk, that Saddam Hussein would pass weapons or materials or information to terrorist networks,” Bush said in a speech in Wilkes Barre, Pa. “In the world after Sept. 11, that was a risk we could not afford to take.”

But Carl Levin of Michigan, the top Democrat on the Armed Services Committee, said Duelfer’s findings undercut the two main arguments for war: that Saddam had weapons of mass destruction and that he would share them with terrorists like al-Qaida.

“We did not go to war because Saddam had future intentions to obtain weapons of mass destruction,” Levin said.

The report also concludes that the Iraqi government was able to manipulate a U.N. oil-for-food program to avoid the sanctions’ effects for a few years, acquiring billions of dollars to import goods such as parts for missile systems. Duelfer accused the former head of the U.N. oil-for-food program of accepting bribes in the form of vouchers for Iraqi oil sales from Hussein’s government.

“Once the oil-for-food program began, it provided all kind of levers for him (Hussein) to manipulate his way out of sanctions,” Duelfer told Congress Wednesday.

He said he believed sanctions against Hussein — even though they appeared to work in part — were unsustainable long term.

On specific points, Duelfer said:

— It is unclear what happened to Hussein’s banned weapons produced before 1991 that he had declared in the 1990s to the United Nations but were never accounted for. Hussein declared having 550 155-mm artillery shells with mustard agents, but it’s not known what became of most of them. Fifty-three “residual rounds” have been found, and the others are not considered a significant threat.

— Aluminum tubes suspected of being used for enriching uranium for use in a nuclear bomb were likely destined for conventional rockets and that there is no evidence Iraq sought uranium abroad after 1991. Both findings contradict statements made by Bush and other top administration officials before the war.

— The likelihood of finding the stockpiles that the president spoke about before the war was “less than 5 percent.”

— The inspectors found no evidence that Saddam was passing weapons of mass destruction material to terrorist groups but acknowledged that wasn’t a strong focus of the report.

Traveling in Africa, British Prime Minister Tony Blair, Bush’s main foreign ally in the war, said the report shows Saddam was “doing his best” to evade the U.N. sanctions.

But the former head of the U.N. weapons inspection team, Hans Blix, said: “Had we had a few months more (of inspections before the war), we would have been able to tell both the CIA and others that there were no weapons of mass destruction (at) all the sites that they had given to us.”