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 U.S. 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 Aug. 24 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 U.S. 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 support the U.S., 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 either.
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.
Hisham Jaber, a retired Lebanese army general who closely follows Syria's war, said it would be "political suicide" for the regime to commit such an act given Obama's warning. He also questioned U.S. assertions that the Syrian rebel fighters could not have launched sophisticated chemical weapons. He said that some among the estimated 70,000 defectors from the Syrian military, many of them now fighting for the opposition, could have been trained to use them.
"It is conceivable that one or more know how to fit a rocket or artillery shell with a chemical agent," said Jaber, who also heads the Beirut-based Middle East Center for Studies and Political Research. He claimed Syrian insurgents have acquired chemical weapons, bought from tribes in Libya after the fall of dictator Moammar Gadhafi, through Saudi interlocutors. Other weapons from Libya have been used in the conflict, though Jaber did not offer evidence to support his chemical weapon claim.
Saudi Arabia has been a chief supporter of the opposition. Prince Bandar bin Sultan, head of Saudi intelligence, recently flew to Moscow, reportedly on a mission to get Russia to drop its support for Assad.
Syrian government officials and Assad accused foreign fighters of carrying out the attacks with the help of countries like Saudi Arabia and Turkey in the hopes of prompting an international military intervention.
Syria says some of its own soldiers were badly contaminated in Jobar, on the edge of Damascus, as they went into tunnels cleared by the rebels. U.N. experts, who had been collecting tissue and other samples from victims in Ghouta, also visited the Mazzeh military hospital in Damascus, taking samples from injured soldier there.
Two days after the Ghouta attack, state television broadcast images of plastic jugs, gas masks, medicine vials, explosives and other items that it said were seized from rebel hideouts. One barrel had "made in Saudi Arabia" stamped on it.
In the U.S., the case for military action has evoked comparisons to false data used by the Bush administration about weapons of mass destruction to justify the 2003 invasion of Iraq.
Multiple U.S. officials have told AP that the intelligence tying Assad himself to the Aug. 21 attack was "not a slam dunk" -- a reference to then-CIA Director George Tenet's insistence in 2002 that U.S. intelligence showed Iraq had weapons of mass destruction -- intelligence that turned out to be wrong. They cite the lack of a direct link between Assad and the chemical assault -- a question the administration discounts by arguing Assad's responsibility as Syria's commander in chief. A second issue is that U.S. intelligence has lost track of some chemical weaponry, leaving a slim possibility that rebels acquired some of the deadly substances.
Russian President Vladimir Putin -- a staunch ally of Assad -- said if there is evidence that chemical weapons have been used, specifically by the regular army, it should be submitted to the U.N. Security Council.
"And it ought to be convincing. It shouldn't be based on some rumors and information obtained by intelligence agencies through some kind of eavesdropping, some conversations and things like that," he told the AP in an interview late Tuesday.
David M. Crane, an international law professor at Syracuse University in New York, said the scale of the attack makes it very unlikely that anyone other than the regime was behind it.
"I think it was a calculated risk by the Assad regime to push to see how far he can go while causing a great deal of political disruption," he said. "It's a huge gamble, but he's in a very risky situation."