We've seen the video proof of the outcome of those attacks.
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.There is every reason to believe that ongoing "expert analysis" will confirm that the videos cited by McDonough show victims of some form of chemical warfare. The experts may even be more specific as to the chemicals themselves. They will not, however, say anything about how those attacks came to pass: Who deployed the weapons, and how did they get them to deploy.
It is because of that final point that I have to wonder if, in making that final sentence, McDonough knows anything about how intelligence does work. For example, neither judicial decisions nor the interpretation of intelligence evidence can be based strictly on abstract principles of logic. The concept of reasonable doubt was introduced in recognition of the inadequacy of pure logic. Any case made in court can never be anything more than a well-formulated hypothesis. It is up to subjectivity to then decide whether or not that hypothesis should be rejected on the basis of reasonable doubt..
Having had the opportunity to examine some training material for intelligence analysts, I can state that the way in which intelligence "works" is not that different. The most important difference is that, while in most courts, there are only two hypotheses at stake, those of the prosecution and the defense, respectively, intelligence analysis needs to take a broader point of view. Thus, in the face of a particularly hostile act, disciplined analysis is required to formulate as many viable hypotheses as the imagination will entertain and then consider all of them in the face of available evidence. There are even systematic techniques through which hypotheses can be "scored" with respect to the evidence. These techniques can just as easily lead to a search for more hypotheses (if no hypothesis is conclusively better than others) as to the selection of the most viable hypothesis.
This is a process that can be just as time-consuming as the analysis required to determine whether a particular location was the site of a chemical attack. It probably also involves more creative imagination (which, itself, can be time-consuming) in the interest of building a sufficiently large "space" of different hypotheses. My guess is that neither McDonough nor the members of the press he was briefing realize the extent to which this is how intelligence "works." However, if we end up barreling into Syria on the basis of ignorance, it would be better for the press to devote some attention to where that ignorance resides.