As it lays the groundwork for a potential military strike against Syria, the Obama administration says it is all but certain that President Bashar Assad used chemical weapons against his own people last week.
Secretary of State John Kerry made the case during a speech at the State Department on Monday.
"We know that the Syrian regime maintains custody of these chemical weapons," Kerry said. "We know that the Syrian regime has the capacity to do this with rockets. We know that the regime has been determined to clear the opposition from those very places where the attacks took place. And with our own eyes, we have all of us become witnesses."
On Tuesday, White House spokesman Jay Carney reiterated the point, saying that "anyone who approaches this logically" would conclude that Assad is responsible for last week's chemical weapons attack near Damascus that reportedly left hundreds dead and thousands more injured.
So, is it possible the United States and its allies are wrong? Is it possible that it was the rebels, or another group within Syria, that launched the reported attack?
"I have been asking myself the same question ever since it happened, because it was difficult to find a rationale [for an Assad-led attack]," says Gwyn Winfield, the editorial director of CBRNe World, a magazine that covers biological and chemical weapons for the industry.
"[A rebel attack] is feasible, but not particularly likely," said Winfield.
What Winfield means is that this seems like a lose-lose situation for Assad. A chemical attack by the regime would seem to bolster the opposition, because it could mean an international intervention. As for the rebels, there are huge questions about whether they could have pulled off such an attack.
Back in 2002, research conducted by George Lopez, a professor of peace studies at the University of Notre Dame, cast doubt on the presence of weapons of mass destruction in Iraq. In this situation, Lopez rejects the notion that it was the Syrian rebels who used chemical weapons.
Lopez and Winfield agree that the rebels may have the motivation to use chemical weapons.
"This anarchic, killing stalemate" could motivate anyone, Winfield says, but such a scenario just doesn't make sense.
For one thing, the alleged chemical attack happened in the Ghouta region of Damascus. It is controlled by the rebels, and civilians in the area sympathize with the rebels.
"The smart thing [for the rebels] would be for you to aim for barracks and maime/kill a significant few hundred soldiers as the best chance for reverberations that played to your advantage," said Lopez. "This was not done."
It seems clear, Lopez says,"that some armed unit foot soldiers were sent in by Assad some time after the attack in limited numbers. That achieved the desired effect of making the case that since Assad soldiers were hit, the weapons came from the 'terrorists;' but these were exemplars, too few to make a strategic difference for the rebels."
In making the case against Assad, the U.S. has said it is his forces who have the capabilities to launch such an attack and that the rebels do not.
An August 20 report by the Congressional Research Service (pdf) says that Syria has had a vast stockpile of chemical weapons since the early 1980s and perhaps as far back as 1973. Not only that, but the military was trained by the Soviets and possesses the delivery methods — scud missiles and batteries of rocket launchers — that could be used to "rapidly achieve lethal doses of non-persistent agents in a concentrated area."
The report goes on to explain that U.S. officials "have unanimously stated that the weapons stockpiles are secure."
Winfield maintains that the Free Syrian Army has the experience and perhaps even the launching systems to perpetrate such an attack. But that would mean that U.S. officials, and Assad himself, were wrong when they said the chemical stockpiles were secure.
"If [the rebels] have overrun an arms dump which had some of the agent, if a defector brought a limited amount with him, then it would explain why some of the signs and symptoms showed less toxicity than we expected," Winfield said. "That is a lot of 'ifs,' though."
Lopez concurs: "Western intelligence has been standing on its head to monitor all intel about those groups hostile to the West and what they have in their weapons access and supply. The amount of gas agents seemingly used was way beyond what a clandestine group could mix and develop without detection. And it is unclear they would have the expertise to mix the agents.
"Is it possible that a rebel group overran a storage facility of the government and captured some shells that were ready to be activated and then did so?" Lopez says. "Yes, but it would have had to have been a very large seizure preceded by a big battle between Assad top teams and rebels. It could not have happened without inside/outside knowledge."
All of that said, note that the U.S. has qualified every statement it has made about the situation. Kerry said it is "undeniable" that chemical weapons had been used in Syria and he set out a case against Assad without directly blaming the regime for the attack.
During his daily press briefing Tuesday, Carney said: "There is also very little doubt, and should be no doubt for anyone who approaches this logically, that the Syrian regime is responsible for the use of chemical weapons on August 21st outside of Damascus."
Jean Pascal Zanders, who worked for the European Union Institute for Security Studies from 2008 to 2013 and concentrated on the non-proliferation of chemical weapons says until the U.N. investigative team presents its report, "we need to keep our minds open that the events of last Wednesday could in whole or partially have alternative explanations."
"In fact, we – the public – know very little beyond the observation of outward symptoms of asphyxiation and possible exposure to neurotoxicants, despite the mass of images and film footage," Zanders added. "For the West's credibility, I think that governments should await the results of the U.N. investigation."