Interactive Grid: Keeping track of the conflict in Syria through videos, images and tweets.

Despite months of laboratory testing and scrutiny by top U.S. scientists, the Obama administration’s case for arming Syria’s rebels rests on unverifiable claims that the Syrian government used chemical weapons against its own people, according to diplomats and experts.

The United States, Britain and France have supplied the United Nations with a trove of evidence, including multiple blood, tissue and soil samples, that U.S. officials say proves that Syrian troops used the nerve agent sarin on the battlefield. But the nature of the physical evidence — as well as the secrecy over how it was collected and analyzed — has opened the administration to criticism by independent experts, who say there is no reliable way to assess its authenticity.

The technical data presented by the three Western powers is of limited value to U.N. inspectors trying to determine whether
Syria’s combatants used chemical weapons during the country’s 25-month-old conflict. Under the United Nations’ terms of reference, only evidence personally collected by its inspectors can be used to fashion a final judgment.

But no inspectors have been allowed inside Syria, so Western governments have relied on physical evidence smuggled out of the country by rebels or intelligence operatives. Precisely who acquired the evidence and what methods were used to guard against tampering may be unknowable, according to experts experienced at investigating chemical weapons claims.

“You can try your best to control the analysis, but analysis at a distance is always uncertain,” said David Kay, a former U.N. weapons inspector who led the U.S. search for weapons of mass destruction in Iraq after the 2003 invasion. “You’d be an idiot if you didn’t approach this thing with a bit of caution.”

U.S. defends analysis

The Obama administration announced last week that it was expanding military support to the rebels after concluding with “high confidence” that Syria’s government had used chemical weapons on a small scale, according to a White House statement. President Obama had warned Syrian leader Bashar al-Assad in August that any use of his chemical arsenal would cross a “red line” and draw a strong U.S. response.

The first report of sarin attacks trickled out of Syria in January, and the administration initially played it down. By March, Britain and France reported that they had received evidence of such attacks and, in a joint letter, asked the United Nations to investigate rebel claims of chemical weapons use by Syrian authorities. Britain also provided soil samples it had tested. The Obama administration continued to collect and analyze data for three more months before reaching the same conclusion.

The number of deaths from poison gas was estimated at 100 to 150 — a relatively small number in a conflict that has killed more than 90,000 people. But the “red line” declaration had committed the administration — which had been resisting pressure to militarily intervene — to act. At the same time, the president’s language handed the Syrian opposition a powerful incentive to fabricate evidence, some weapons experts noted.

“If you are the opposition and you hear” that the White House has drawn a red line on the use of nerve agents, then “you have an interest in giving the impression that some chemical weapons have been used,” said Rolf Ekeus, a Swedish scientist who headed up U.N. weapons inspections in Iraq during the 1990s.

U.S. officials staunchly defend what they describe as an extensive, rigorous and multilayered analysis that led to the White House’s June 13 pronouncement on the use of chemical weapons in Syria. The conclusion that Assad’s forces used sarin was based on scientific assessments of dozens of evidentiary samples representing multiple attacks spanning several months, according to two U.S. officials familiar with the evidence.

The evidence came from a variety of sources, and some was collected by non-Syrians, said the sources, who like others interviewed for this report spoke on the condition of anonymity, citing strict secrecy surrounding the operation. Details about how the evidence was gathered and tested could not be disclosed without compromising ongoing intelligence operations, the officials said.

Having been famously burned by the 2003 intelligence failure over Iraq’s alleged weapons of mass destruction, U.S. analysts now approach all such claims with exceptional care, said a third administration official familiar with intelligence analysis. “You have to use sophisticated analytic techniques that account for, and carefully weigh, competing evidence and subject your findings to intense self-imposed scrutiny,” the official said.

But Western officials and diplomats also acknowledged that the lack of transparency undermined the credibility of the chemical-weapons claims.

“The chain-of-custody issue is a real issue,” said a senior Western diplomat whose government has closely tracked the Syrian investigation. But the official said the totality of the evidence “from different sources, different times, different locations” should convince the U.N. investigating body that the claims were real.

Challenge for U.N.

The key fact-finder for determining whether and how chemical weapons were used is Ake Sellstrom, a Swedish scientist and the chief U.N. weapons inspector. He must try to link the physical evidence to a verifiable use of chemical weapons inside Syria, like a criminal prosecutor assembling a case that can stand up in court.

Sellstrom is scheduled to travel next week to Turkey, the first leg in a reporting trip that will include stops in Lebanon and Jordan. There, he is expected to interview Syrian witnesses and medical professionals who claim to have treated victims of chemical-
weapons attacks.

The senior Western diplomat said Sellstrom and his team should be able to put together a “pretty coherent picture of what happened,” though the diplomat voiced frustration that it has taken Sellstrom months to send a team to the region to interview refugees.

“If we have criticism with Sellstrom, it’s that he has been very passive,” the official said.

U.N. officials continue to push for an on-the-ground inspection in Syria, even as they acknowledge the diminishing chances that the Assad government will let them in.

“The validity of any information cannot be ensured without convincing evidence of the chain of custody,” U.N. Secretary General Ban Ki-moon cautioned after the White House disclosed plans to arm Syria’s rebels. “That is why I continue to emphasize the need for an investigation on the ground in Syria that can collect its own samples and establish the facts.”

But even if inspectors are allowed in, the passage of time since the alleged attacks would make their task even more difficult, as sarin degrades quickly after exposure to air and sunlight, weapons experts say.

Jean Pascal Zanders, who until recently was a research fellow at the European Union Institute for Security Studies, said he has scoured the Internet for photographs, video and news reports documenting alleged nerve agent attacks in Syria. What he has seen has made him a skeptic.

Few of the photographs, Zanders said, have borne the trademark symptoms of a chemical weapons attack. In a paper he presented last week to the E.U. Non-Proliferation Consortium, he compared photographs documenting Iraq’s 1998 chemical weapons attack against Kurds in the town of Halabja.

The Halabja victims appeared to have died instantaneously from chemical agents, he said, and their bodies showed telltale signs of exposure to sarin: blue lips and fingertips caused by suffocation and a pink hue brought on by excessive sweating and high blood pressure. “No press reports from Syria refer to those descriptions, which is one of the reasons why I am skeptical about those reports,” he said.

Zanders said the problem with the U.S., British and French evidence is that it cannot be tested by independent scientists. Some of the published reports of chemical weapons use “make certain alarm bells ring,” he said, but it is impossible to reach a definitive conclusion on the basis of what governments have put forward. “We don’t have the barest of information. There is not even a fact sheet documenting the samples,” he said. “This is an immensely political process, and there is no way of challenging the findings.”

Other weapons experts were prepared to accept that sarin was used but said the allegation that the Syrian government has deliberately used toxins against its own people appeared to based on circumstantial evidence.

“There are so many people who would like us to believe that the regime used chemical weapons,” said a former senior U.S. official who had been involved in intelligence assessments of claims about weapons of mass destruction. “You have to question whether any of those advocates were involved in collecting the evidence.”

Warrick reported from Washington.