As the Obama administration fights for political and diplomatic support for attacking the regime of Bashar al-Assad over chemical weapons in Syria, one critical issue remains unclear.

Who, exactly, used these weapons of mass destruction to kill hundreds of Syrians?

So far, no slam-dunk, smoking-gun evidence – the kind that proved to be so elusive in Iraq 10 years ago – has been produced. That has left the door open to alternate scenarios and conspiracy theories about who was responsible – the Assad regime or rebel groups backed by outside forces.

In his initial statement 10 days ago about the use of chemical weapons in Syria, Secretary of State John Kerry was adamant about who was to blame for an attack in the suburbs of Damascus that he said killed 1,429 people, including 426 children.

“We know where the rockets were launched from, and at what time,” Secretary. Kerry said, citing but not detailing intelligence reports. “We know where they landed, and when. We know rockets came only from regime-controlled areas, and went only to opposition-controlled or contested neighborhoods.”



“We know that a senior regime official who knew about the attack confirmed that chemical weapons were used by the regime,” Kerry added.

In blasting what she called a "blatant violation of international law, a war crime, and a crime against humanity,” European Union foreign-policy chief Catherine Ashton also pointed a finger at President Assad for the Aug. 21 chemical attack.

"[The Syrian government] is the only one that possesses chemical weapons agents and the means of their delivery in a sufficient quantity," she said Saturday.

Some members of Congress have received classified intelligence briefings, presumably including evidence the Obama administration knows it needs to provide if it’s to win congressional authorization for the use of US military force in Syria.

But publicly, at least, the White House has yet to make its case in any detail, and its latest comments haven’t clarified things.

On Sunday, White House chief of staff Denis McDonough said a "common-sense test" rather than "irrefutable, beyond-a-reasonable-doubt evidence" makes the Syrian government responsible.

"We've seen the video proof of the outcome of those attacks,” Mr. McDonough said on CNN’s “State of the Union.”

“All of that leads to a quite strong common-sense test irrespective of the intelligence that suggests that the regime carried this out,” he said. “Now do we have a picture or do we have irrefutable beyond-a-reasonable-doubt evidence? This is not a court of law and intelligence does not work that way. So what we do know and what we know the common-sense test says is [Assad] is responsible for this. He should be held to account."

Part of the problem for President Obama is that showing US evidence in greater detail could reveal sources and methods of intelligence gathering – a problem all administrations have faced over the years, whether it has to do with signals gathering and code breaking, satellite photos, or spies on the ground.

There is open-source evidence that provides clues about the attack, including videos of the rockets that analysts believe were likely used. US officials on Saturday released a compilation of videos showing victims, including children, exhibiting what appear to be symptoms of nerve gas poisoning.

Some experts think the size of the strike, and the amount of toxic chemicals that appear to have been delivered, make it doubtful that the rebels could have carried it out, The Associated Press reports. What's missing from the public record is direct proof, rather than circumstantial evidence, tying this to the regime.

The Obama administration says its assessment is based mainly on satellite and signal intelligence, including indications in the three days prior to the attack that the regime was preparing to use poisonous gas, but multiple requests to view that satellite imagery have been denied, according to the AP.

The Obama administration maintains it intercepted communications from a senior Syrian official on the use of chemical weapons, but requests to see that transcript have been denied. So has a request by the AP to see a transcript of communications allegedly ordering Syrian military personnel to prepare for a chemical weapons attack by readying gas masks.

That’s given skeptics and conspiracy advocates an opening.

“We are unaware of any reliable evidence that a Syrian military rocket capable of carrying a chemical agent was fired into the area,” writes former CIA analyst Ray McGovern, a political activist and frequent critic of US policies. “In fact, we are aware of no reliable physical evidence to support the claim that this was a result of a strike by a Syrian military unit with expertise in chemical weapons.”

“There is a growing body of evidence from numerous sources in the Middle East – mostly affiliated with the Syrian opposition and its supporters – providing a strong circumstantial case that the August 21 chemical incident was a pre-planned provocation by the Syrian opposition and its Saudi and Turkish supporters,” Mr. McGovern writes on documentary filmmaker Michael Moore’s website. “The aim is reported to have been to create the kind of incident that would bring the United States into the war.”

That’s essentially the claim made by journalists Dale Gavlak (who has reported for the AP, the BBC, and NPR) and Yahya Ababneh on MintPressNews.com headlined “EXCLUSIVE: Syrians In Ghouta Claim Saudi-Supplied Rebels Behind Chemical Attack.”

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But such reports, based at least partially on hearsay, seem without solid evidence, and they follow essentially the same line promoted by the Assad regime and its principal patron, Russia.

United Nations investigators are expected to make their findings public in about two weeks. At this point, it’s unclear whether that will support Mr. Obama’s assertions as the administration proceeds with plans to attack Syria.