Beirut - The US government insists it has the intelligence to prove it, but the American public has yet to see a single piece of concrete evidence - no satellite imagery, no transcripts of Syrian military communications - connecting the government of President Bashar Assad to the alleged chemical weapons attack last month that killed hundreds of people.
In the absence of such evidence, Damascus and its ally Russia have aggressively pushed another scenario: that rebels carried out the21 August 21 chemical attack. Neither has produced evidence for that case, either. That's left more questions than answers as the US threatens a possible military strike.
The early morning assault in a rebel-held Damascus suburb known as Ghouta was said to be the deadliest chemical weapons attack in Syria's 2½-year civil war. Survivors' accounts, photographs of many of the dead wrapped peacefully in white sheets and dozens of videos showing victims in spasms and gasping for breath shocked the world and moved President Barack Obama to call for action because the use of chemical weapons crossed the red line he had drawn a year earlier.
Yet one week after Secretary of State John Kerry outlined the case against Assad, Americans - at least those without access to classified reports - haven't seen a shred of his proof.
There is open-source evidence that provides clues about the attack, including videos of the rockets that analysts believe were likely used. Some experts also 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.
Searching for support
The Obama administration, searching for support from a divided Congress and sceptical world leaders, says its own 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, though the administration produced copious amounts of satellite imagery earlier in the war to show the results of the Syrian regime's military onslaught. When asked on Friday whether such imagery would be made available showing the 21 August incident, a spokesperson referred The Associated Press to a map produced by the White House last week that shows what officials say are the unconfirmed areas that were attacked.
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.
The US administration says its evidence is classified and is only sharing details in closed-door briefings with members of Congress and key allies.
The assessment, also based on accounts by Syrian activists and hundreds of YouTube videos of the attack's aftermath, has confounded many experts who cannot fathom what might have motivated Assad to unleash weapons of mass destruction on his own people - especially while UN experts were nearby and at a time when his troops had the upper hand on the ground.
Rebels who accuse Assad of the attack have suggested he had learned of fighters' plans to advance on Damascus, his seat of power, and ordered the gassing to prevent that.
"We can't get our heads around this - why would any commander agree to rocketing a suburb of Damascus with chemical weapons for only a very short-term tactical gain for what is a long-term disaster," said Charles Heyman, a former British military officer who edits The Armed Forces of the UK, an authoritative bi-annual review of British forces.
Inconsistencies over the death toll and other details related to the attack also have fuelled doubts among sceptics.
The Obama administration says 1 429 people died in 12 locations mostly east of the capital, an estimate close to the one put out by the Western-backed Syrian National Coalition. When asked for victims' names, however, the group provided a list of 395. On that list, some of the victims were identified by a first name only or said to be members of a certain family. There was no explanation for the hundreds of missing names.
In Ghouta, Majed Abu Ali, a spokesperson for 17 clinics and field hospitals near Damascus, produced the same list, saying the hospitals were unable to identify all the dead.
Lower casualty stats
Casualty estimates by other groups are far lower: The Britain-based Syrian Observatory for Human Rights says it only counts victims identified by name, and that its current total stands at 502. It has questioned the US number and urged the Obama administration to release the information its figure is based on. The AP also has repeatedly asked for clarification on those numbers.
The humanitarian group Doctors Without Borders says it has not been able to update its initial 24 August estimate of 355 killed because communication with those on the ground around Damascus is difficult. That estimate was based on reports from three hospitals in the area supported by the group.
Moreover, the group, whose initial report was cited in US and British intelligence assessments, has rejected the use of it "as a justification for military action," adding in a disclaimer published on its website that the group does not have the capacity to identify the cause of the neurotoxic symptoms of patients nor the ability to determine responsibility for the attack.
French and Israeli intelligence assessments back the US, as does reportedly Germany's spy agency, on its conclusion the Syrian regime was responsible. However, none have backed those claims with publicly presented evidence.
Some have suggested the possibility, at least in theory, that the attack may have been ordered by a "rogue commander" in Assad's military or fighters seeking to frame the regime.
Testifying on Wednesday before the House Foreign Affairs Committee, Defence Secretary Chuck Hagel rebuffed a congressman's bid to declassify one of the key pieces of intelligence Kerry publicly cited last week: intercepted communications telling Syrian military units to prepare for the chemical strikes.