And here's an excerpt from a letter that the White House sent to Sen. Carl Levin (D-Mich.), explaining what U.S. intelligence agencies do and do not know. I've bolded some parts for emphasis:
This assessment is based in part on physiological samples. Our standard of evidence must build on these intelligence assessments as we seek to establish credible and corroborated facts. For example, the chain of custody is not clear, so we cannot confirm how the exposure occurred and under what conditions. We do believe that any use of chemical weapons would very likely have originated with the Assad regime. Thus far, we believe that the Assad regime maintains custody of these weapons, and has demonstrated a willingness to escalate its horrific use of violence against the Syrian people.
The possibility that Syrian regime forces may have used chemical weapons is too potentially horrific to not take as seriously as possible. But, particularly after the Iraqi weapons of mass destruction that never emerged, it is important to treat this potential development with the necessary degree of caution.
To that end, here are three caveats to keep in mind as the story develops. To be clear, none of these means that the U.S. intelligence assessments are necessarily wrong, and, indeed, the administration's description of the intelligence has expressed a clear degree of uncertainty. But these are the sorts of issues and uncertainties to keep in mind as more information comes out.
1. There's a difference between "exposure" and "use" of chemical weapons.
The letter to Levin and McCain cites "physiological evidence" of "exposure" to sarin gas. To be clear, that's not the same thing as, say, a first-hand report of Syrian troops dumping the gas on rebels. It does not prove how they were exposed.
Imagine, if you'll forgive the grim metaphor, that a friend of yours is in an emotionally abusive relationship. You know she and her husband argue frequently, and you don't trust her husband, who's a big guy. You've warned him that physical violence is a red line, that if he hits her you'll get the police to intervene. Then, one day, you notice your friend has bruises on her arm. You can infer that he may well have hit her, but you can't know for sure without more evidence.
It's possible, however unlikely it sounds, that these individuals were exposed to sarin gas through something other than a deliberate attack by regime forces. As nonproliferation expert Jeffrey Lewis writes on Twitter, "one wants to be sure victims weren't exposed ransacking a captured CW site, etc."
Given that the White House went out of its way to note that it's not sure about the chain of custody of the evidence, this question is a particularly important one.
2. Using a "small amount" of sarin doesn't make strategic sense, given the risks.
The United States has made quite clear that it considers chemical weapons use a red line, and, regardless of how you feel about the likelihood that the Obama administration would intervene if that line was crossed, the Assad regime had to take the cost-benefit into consideration. Libyan leader Moammar Gaddafi did not fare well against the Western intervention in his own country. So, even if the risk of sparking an intervention is small, presumably Assad forces would do it only if they really, really thought they needed to. And it's hard to imagine that using a "small amount" of chemical weapons would do the regime forces enough good to merit the risk.
As chemical weapons expert Ralf Trapp told Foreign Policy's John Hudson, "From a military perspective, it doesn't make sense to use chemical weapons bit by bit. ... Why would the regime just put it on a grenade here or a rocket launcher there? It's just not the way you'd expect a military force to act."
3. Where exactly is the U.S. red line?
In August, President Obama said, using language with less than lawyerly precision, "We have been very clear to the Assad regime, but also to other players on the ground, that a red line for us is we start seeing a whole bunch of chemical weapons moving around or being utilized. That would change my calculus."
If and when we do know exactly what may have happened to expose individuals to sarin gas, it may not be crystal clear whether Obama's red line was crossed. If it was an accidental spill, for example, does that count as crossing the line because it suggests Assad's forces are losing control of their enormous chemical weapons stockpile? What if the spill happened because the forces were repositioning the weapons closer to a front line but did not deploy them? What if the weapons were used deliberately, but by a rogue commander who was punished for using them without authority?
Whatever more we learn about the sarin exposure, it's entirely possible that the facts of what happened will remain unclear -- and so may the intentions behind any responsible individuals. But just as there is a real urgency to investigate and understand what happened, so, too, is it important to retain a degree of caution.