As Congress debates a resolution authorizing the use of force in Syria, the inspection of alleged chemical attack sites by United Nations experts has already been relegated by Washington to a historical footnote.

The Obama administration has asserted that the findings — expected in less than two weeks — no longer matter, citing its own evidence that the Syrian government was behind the chemical weapons attack last month in the Damascus suburbs. Few lawmakers have pressed the administration to wait for the inspectors to release their results.

Weapons experts, however, say such claims miss a fundamentally important truth about the U.N. inspection process: Its on-the-ground analysis can confirm key forensic details about the attack, possibly bolster the case against the Syrian government and help rally international support for action.

“I would strongly encourage the Obama administration and any other countries considering a military strike to wait for the results of the U.N. secretary general inspection,” said Amy Smithson, an expert on chemical and biological warfare at the Monterey Institute of International Studies. “If the U.N. analysis of samples turns out to reveal the use of classic warfare agents believed to be in the Syrian government’s arsenal, like sarin and VX, then the neon light already pointed towards [President Bashar al-Assad] glows even brighter. The point is: Don’t count the inspectors out.”

On Thursday, President Obama heads to a Group of 20 summit outside St. Petersburg, where Russian President Vladimir Putin is expected to press Washington to await the findings of the U.N. inspections before taking any action. On the president’s stopover Wednesday in Stockholm, the Swedish prime minister made his own subtle plea for patience, saying, “Let’s put our hope into the United Nations.”

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The United Nations has said its chief weapons inspector, Ake Sellstrom, and his team are working through the night to assess witness accounts and laboratory tests on samples from the first chemical weapons investigations ever conducted on Syrian soil. U.N. Secretary General Ban Ki-moon — expressing concern that military action would worsen the violence — also has urged the United States to give the inspectors time.

The Obama administration has rejected such calls, noting that U.N. experts have not been asked to determine who carried out the alleged chemical weapons attack. Rather, they are expected to establish whether chemical weapons were used — a question the administration thinks the U.S. intelligence community has answered.

“The U.N. investigation will not affirm who used these chemical weapons,” Secretary of State John F. Kerry said recently. “By the definition of their own mandate, the U.N. can’t tell us anything that we haven’t shared with you this afternoon or that we don’t already know.”

The United States has long been among the foremost champions of U.N. inspections. Hans Blix, who led U.N. inspections in Iraq during the Saddam Hussein era, said the United States has for decades served as one of the world’s greatest political and financial supporters of the inspections process. But its enthusiasm has waned when inspections have complicated U.S. military objectives.

In Iraq, he recalled, the George W. Bush administration provided U.N. inspectors with information about three dozen suspected weapons sites and underwrote the cost of running U-2 spy flights for the United Nations. But when President Bush decided to go to war, he dumped the U.N. inspectors.

“They were helpful until the tilt came,” Blix said. “At the last meeting I chaired before the invasion, the U.S. representative said all this detailed examination . . . is useless. What we needed to see was change of mind, a sort of conversion of Saddam Hussein.”

In the case of Syria, Washington has shown a similar ambivalence toward inspections.

The United States has used its influence to press the Syrian government to grant inspectors greater powers. Hours after reports emerged indicating a large number of civilians had been asphyxiated by a toxic gas last month, U.S. diplomats in New York introduced a statement demanding that Syria admit U.N. inspectors into the area to collect evidence.

Less than a week later, however, the Obama administration called on the U.N. inspection team to leave, saying Syria had shelled the sites in question for days, making conditions for the inspectors too dangerous and contaminating evidence that would have made the process credible.

In interviews, though, former U.S. and U.N. inspectors say that it is highly unlikely that Syria could have completely scrubbed away evidence of a chemical weapons attack. Many said the U.S. argument was unconvincing.

“I don’t think that’s a valid objection to U.N. inspections,” said Blix, who added that he is personally inclined to believe that the Syrian government did use chemical weapons against its own people. “I agree that sarin is a volatile substance and much of it has blown away. But that’s not the only source of information. There are interviews with eyewitnesses, victims’ body fluids and urine. This is something that stays around for a fair amount of time.”

Charles Duelfer, a former U.N. inspector who headed the CIA’s Iraq Survey Group, said he finds it “ironic” that the Obama administration finds itself on similar ground as the Bush administration, which also decided to bypass the work of U.N. inspectors in Iraq.

“It’s odd that they are dismissing the United Nations,” he said.

“By going through the U.N., you subject yourself to the vagaries of that schedule,” Duelfer said. “Certainly it will take time for inspectors to develop results of the analysis of these samples. But the utility of that is it is seen as credible in the international community.”

Duelfer acknowledged that it’s possible that U.N. inspections could provide a muddy picture of what happened in Syria. Some of the evidence, according to diplomatic officials, may point to exposure of Syrian security forces — Russia and Syria could claim that it proves rebel forces used chemical weapons, but British, French and American officials could contend that it was the result of misfired rockets hitting the government’s own troops.

Still, Duelfer disputed the U.S. contention that it is too late to collect credible evidence. The science of inspections, he said, has advanced so that traces of chemical weapons agents can be obtained years after an attack.

In 1992, four years after Hussein gassed Kurdish civilians in the town of Halabja, the Boston-based Physicians for Human Rights collected soil samples from the area and sent them to the Chemical and Biological Defense Establishment of the British Defense Ministry. The tests found traces of sarin and mustard gas.

“If you believe there should be a U.N. mechanism for investigating these things, anytime they get in they will get data that’s useful,” Duelfer said. “You may get lucky.”